My mother’s computer slowed down and eventually became unusable after upgrade to Windows XP SP3. One option, of course, was to remove SP3, but we couldn’t even get things working well enough for a reinstallation. (Note that I’m typing this on an Ubuntu machine.)
Despite a number of things that annoy me about Vista, I don’t think it’s actually a bad operating system. So I suggested to my brother (who was doing all the work) that we install Vista Home Premium, which would duplicate her Media Center Edition features.
Unfortunately upgrade was disabled. We were unable to determine precisely why, but we were able to do a clean install and preserve her files. A couple of instruction sets on the internet were unclear, but even without being able to log into the current windows, and with upgrade disabled, we were able to do an install without format, and old files were copied into WINDOWS.OLD. The new installation is doing quite well, though we’re still working on audio drivers.
Follow the prompts very closely so that you don’t accidentally choose to reformat your drive, and it will work. We were using a full version, not an upgrade.