Does your PC have a WiFi card built in? If it does you night be able to create an AdHoc Wifi connection without a router.. http://unlockforus.blogspot.com/2008...-hoc-wifi.html

Getting back to the USB problem. Have you tried connecting the phone during reboot?
You can try connecting in PC Suite mode, or If all else fails reflash your N900. Maemo will be back to factory reset, but your mails, call history, SMS messages and files should still be intact on the eMMC and MyDocs.