Oh, you could also try going into your Windows partition, and renaming the explorer.exe to something like explorer.bak
Once you do that, put in your Windows installation disc, and go into start-up repair. I can't guarantee that will work, but it's worth a shot.
If it doesn't, you can probably find a copy online or something as long as it ain't customized for each PC it's installed on.
Just remember, nothing is lost. So really, there's no big reason to panic or flip out. If you can't get it working with any of the above, then you can put in the Ubuntu live CD, and boot into Live mode (not install). Then run gparted, create a new partition, and install Windows again. Mount your partitions (I think they're available in "Computer"), and copy over anything you need.
If you are sticking with Linux though, my first reply is probably better.