Some viruses on the desktop can mess with removable media (your N900 when plugged into usb).

Run a good virus scanner on your PC with your N900 plugged in (may I suggest something like AVG free), you may have caught a virus somewhere.

Check your N900 for files like "autorun.inf". The default autorun.inf only has a section for a custom icon and nothing else. If you find an autorun.inf file that has an "open=" command in it, post it back here.

Good luck!