Unfortunately, even after you scanned and healed your PC, the virus still has done damage to your n900's data partition.

Flashing the EMMC is the best and safest bet, but that would return your device to stock condition.
If you're okay with that (as it is the safest course of action), check this guide: http://wiki.maemo.org/Updating_the_tablet_firmware

If you're having problems with the device not being detected, try rebooting your N900 and as soon as the screen goes black press and hold "u" on the keyboard until you see a little USB icon appear on your N900's screen.

Good luck!