My Vista upgrade tips:
1. Run the Vista upgrade advisor. If anything is flagged as unsupported, uninstall, unplug, or remove it from your PC, reboot, and run the advisor again. Repeat until you're reasonably free of anything that might have problems.
2. Once you've started the Vista installer, don't touch the PC. Just leave it alone. I'm guessing continuing to use IE freaks it out just a little bit.
3. Finally, get ready to manually download and install a bunch of driver and other updates. Microsoft Update doesn't know about most of the good stuff yet, and unless you update your video drivers (for example), you might not be able to play games and such.
4. Oh, and don't even think about upgrading unless you have a fancy video card and 2 GB of RAM. Honest.