I have a Nokia N800 that is stuck at the Nokia startup screen when you first power it on. It stays stuck for about 30 seconds and then shuts off. I would try to flash the firmware,but when I connect the usb to the computer and hold the home button + power button it doesn't connect and the usb logo doesn't appear on the N800.

Hmm, computer?
A detailed description of system!

Linux? Windows (connectivity drivers installed?)
original USB cable? Other USB port (not a front hub)
And so on and so on and so on ...

About the N800 and its willingness to be flashed I do not know much. So possibly wait for endsormeans to appear or try the search.

Another thought, maybe the battery is on its last leg? The device starts booting, realizes the battery is nearly dead and shuts down. Plugging the USB should start charging it. If it doesn't, then maybe the cable is dodgy, as my predecessor suggested.

nuh-nuh-nuh-NUH-NUH!
I is here!

1stly the n800 doesn't charge via usb.

next there are 2 things all manageable and easy...
1- it's the battery...either :
A- the battery is dead ...as in it will never hold a significant amount of charge ever again to be able to power the device properly...meaning you need a new battery.
B- the battery needs to be properly charged 1st before actually starting the device up...meaning it's charge has simply drained to a level it will not start up and stay on...but if turned off...plugged in and left alone to charge properly it will be a functional battery again and a functional device.
C- the device does need to "quasi-" turn it's self on during charging and if the battery charge is so low it cannot accomplish that ...a external plugin charger [very cheap on ebay] may be necessary to "jump-start" the very-low battery charge to a level acceptable to be placed back in the device and fully and properly charged from there...

In all ...although it is possible the battery is dead dead...
I doubt the battery is dead dead....merely very low...and needs some proper attention...

2- Highly unlikely ...considering the sitch...but it is possible it needs a reflash too...but yet again I doubt it...since EVERYTHING points to the battery...
After the battery sitch is sorted then you should have a device which will properly start...and stay on...
after that.... assessing the state of the os will be possible...
which very well could have nothing at all wrong with it.

Regarding connecting via usb to your pc....
I have no idea of the pc os you are running and therefore can only
speculate on zero information that either :
A- you aren't running a pc os which will recognize the device and device which will not recognize the pc os.
B- your connector cable is shot.
C- the ports on either device or pc are shot.
D- you aren't holding down on the "home" button 1st...THEN pressing the power on button ...and continuing to hold both down long enough from there...
E- due to the battery charge being so low ...the device shuts down BEFORE the device has time to make it to the "usb connected" symbol even appears..[WHICH IS THE MOST LIKELY AND LOGICAL SITCH WHICH LEADS BACK TO POINT NO.1]
F- any possible multiple combinations of the above.

Ahhh, THAT I didn't know. Thanks for the correction. (Maybe I should get one of those. It sounds like fun.)

Actually pichlo ...
yeah they are great...
where the n900 charges from the micro usb port OR is used for data transfer or peripherals ...but cannot be used for both simultaneously...
the n800 [mini usb port] and n810 [micro usb port] both have separate ports AND separate power ports...making them more versatile than the n900 in that regard.
I love both n800's and n810's for what specific qualities they have and that they can do...
actually ..I haven't posted what I ran into [on ebay] in the buy and sell subforum yet....
but there are a couple of n800's and n810's selling stupid stupid cheap right now...under $20 for auction [before shipping]...they have a few days left...check 'em out...
http://www.ebay.ca/itm/121760661282?...84.m1436.l2649
http://www.ebay.ca/itm/Nokia-N-Serie...item2ca1c5dd44
http://www.ebay.ca/itm/Nokia-N-Serie...item1c5918fc18
Best price I've seen in a very long time...
and of course no point in me mentioning it again in the buy and sell subforum anyway...since I am mentioning it here

The OS is Windows XP and it's neither the battery or the OS as I a have another N800 that I'm able to connect to the PC while using the same battery.

This N800 however gets stuck at the Nokia logo and turns off after 30 seconds. The USB logo also doesn't show up after I hold the home button while turning it on.

Hmm...
well then...
so the self same battery works fine in the 2nd n800...
as well you are using win xp [win 7 works fine too by the way]
sooo...
It is possible it is still the battery...
it may be that your n800 is getting persnickety about it...
The exact same issue has happened to me ...once ...with a particular n810 and a particular battery...a completely different battery solved that issue.
do you have a 2nd [fine] battery to test with?
if not then I would wait until getting a new battery.
before jumping head long into os management which may be unnecessary..
also is it the self same usb cable you are using on both n800's?
if so...that would rule out the cable...
the mini usb port STILL may be THE or A contributing problem on the n800 in question...
or it could be the "home" button not connecting under the pad...when pressed...[even still... *]
But...if you do not have the patience to wait for a 2nd battery...
it very well could be the os...

* Funny thing is..it powers on...but isn't given enough time to register the usb icon before shutting off...which still really points to a battery issue...

Never in my life have I found an os issue which PREVENTED me from properly reflashing the firmware...
there is something else going on here...
process of elimination should do the trick...
and ...
as well...
in part the charger used may be a possible additional culprit...
and so in a similar vein...I thought I should mention...
the best chargers I found which were official nokia chargers....
weren't the more rectangular charger...I found I had all kinds of weirdness the rare time with this model...
http://www.ebay.ca/itm/Home-Wall-AC-...item2a17d1ffb4

but with the rather the blockier more square chargers...
no problem ever...
http://www.ebay.ca/itm/Nokia-AC-3E-w...item3cde7dad07

they do make a north american variant as well...