I had this problem initially on XP. Turned out I was not waiting long enough for the driver to initialize. Once I did that, it connected fine every time afterward.

Try this: reboot your device and your PC. Connect USB cable to PC first, then to N900. Select mass storage, and then let the device sit a while. I found that I kept trying to use mine too soon and that was screwing things up.

The problem appears to be on the Windows side. As an aside, I think Microsoft's handling of USB devices can be asinine. Most of mine are recognized as new devices every single time I plug them in. IMO Windows should set them up ONCE, save the config and then Just Work every time after that.