I at least give it a once over with Spybot and if it is still bad afterwards than I will do the reinstall. What is nasty is the lack of backing data up as the user goes. Thus leaving poor hapless me to dig around to make sure I get everything that is, like you said, spread out from one corner to the other. You don't know how many times I come accross PCs that the user saved files like .doc and .xls in the windows directory, etc. WTH is it doing there!
Unfortunately, I do know. I've had the same experience. Most hapless PC users haven't the foggiest idea why there was a "My documents" folder and a "My Briefcase" folder on the desktop or in the Start menu.
Even more unfortunately, many Windows developers haven't the foggiest idea either, placing their data files where they are convenient for THEM to find while they are doing the development and never bothering to move them out of the App's folder when they publish!
Another fun thing is the multiple copies of the same document in various locations with differing or the same creation date... trying to find which one is the most current across multiple directories is not fun.
Sometimes I have to spend an hour or two looking for user files in unusual places because the owner says "Where's my letters to Aunt Ginny folder? I don't see it?"
Backing up user data on a Mac is as easy as dragging the User's home directory to another drive, thumb drive (if big enough) or burning a copy to a DVD or CD.
I find it to be amazing that in 2007, you can still save documents in a system directory with MS-Windows.
When I back up my MIL's and Wife's laptops, all I do is tar (dar actually) the /home directory, and I know I've saved off all files for all users.