Try downloading the Nokia PC Suite Software (from Nokia website) its normally used just for their phones, but as I also have a N95 phone I have the SW on my PC. I have had lots of problems with USB and PC connection, but found 2 things that worked 1) SWAP USB cable - for some reason some USB cables do not work with my N800, no idea why, but i have 2 of the 5 USB cables on my desk that simply refuse to work with my N800 2) Nokia PC Suite - it allows you to browse your memory cards via the Phone Browser, so you can load files on/off your N800.

I'm going to bet that your stuck with problem 1, assuming when you switched PC's you used the same USB cable.

BT