Could the charge problems be a voltage problem within the PC?
If your BIOS has a voltage viewer or if the motherboard came with a system monitor that displays voltage, check these; could indicate PC power supply problem.

I know it’s obvious but the other thing to try is plug the USB cable into each USB socket on the PC, there could be a failing of the USB hardware on the motherboard. I know I have had component failure like this.

Interesting thought. Windows XP certainly caches snap shots of memory in the Prefetch folder in C:\Windows\Prefetch for fast bootup and loading of programs. It is Ok to delete the contents of this folder and then reboot. Windows will then reload the Prefetch folder with fresh images. The only thing noticeable would be initial bootup / program start will be slower after that thing would be back to normal.

More info http://www.windowsnetworking.com/art...efetch-XP.html

I have no idea if this would help! But if zwer is correct it might

Drivers caching has nothing to do with Prefetch. The way I described it is the only way to remove cached/leftover/wrecked drivers for devices that cannot be recognized without wacko cmd commands and heavy registry editor. It will definitely fix all problems with devices that used to work but the system won't recognize them anymore.

About the "Not charging. Insufficient power" message using Windows XP.
Press [Win]+[Break] (MyComputer properties), select the Hardware tab, then "Device Manger".
Expand "Universal Serial Bus controllers".
Right click on every USB Root Hub and select "Properties", until you find the one with the N900 attached and check in the "Power" tab the Power available and Power Required. Also check in the "Power Management" tab if you allow the computer to turn off the USB port to save power.

I had this same problem (getting the "insufficient power" message) earlier today when I had a low battery, and the following worked for me:
I turned off the device, and then connected it to the usb cable attached to my laptop. I waited a few minutes (the led was blinking meaning it was charging), then disconnected the cable from the N900 and turned the phone back on. Finally, I once again connected the device to my computer (while it was on) and it is now allowing me to charge...

Maybe this has something to do with low battery power?

I think the question was answered on the first page, i had all sorts of problems today and I the phone connected to the wireless and Skype enabled.

Switched the WIFI off and it it worked first time.

I would check what apps you are running and if any data connections are present.