The lack of root is a major obstacle. There are ways around it to do what you want (install remote desktop software ie x11vnc) using something like http://kumatux.org/index.html however, you would also need a version of x11vnc compiled for this method and I can't see it on the application list on that site.

The good news is that there is yet another way of enabling USB mass storage mode. In a similar fashion to using the Ubuntu LiveCD to load Ubuntu onto your PC, you can load what's called the Rescue Initrd onto your N900. Whilst you can probably do this from Vista, it's probably best to use the Ubuntu LiveCD again and then follow these instructions from the wiki.

When I tried this on my device the backlight didn't come on. If you shine a torch on the device you can see the list of options available. You just want to press u and those 2 drives you could see earlier on can now be accessed.

When you're finished pressing r will reboot your phone back into maemo.