I point my finger at Windows XP. Had the very same issues myself.
Same cable works fine on my Win 7 laptop (and indeed my Ubuntu laptop), but Windows XP fails to recognise the device, but it charges OK.

So I go into Device Manager and delete the failed N900 connections, reboot and everything works fine after that...........until it does it again a few weeks later.

Interestingly enough, I had to go through the same shenanigans with my N95-8GB before I got my N900.