1) Repartition the drive so they have a 20-40GB C: drive and partition the remainder for their data on D:.
2) After installing a fresh copy of windows on a freshly formatted drive, I install all their applications (make a list of them before reformatting, so you don't forget details like Adobe Flash Player, Acrobat, etc.). Also do all the windows updates and update all the applications.
3) Move the "My Documents" folder to the D: drive - also go through all the applications and verify that the default saving directories are all on the D: drive.
4) IMMEDIATELY Create a image of the C: partition. I use BartPE/WinPE to boot up outside of windows and "Symantec Ghost" to create an image. You can use any boot disk and any image program you want. Save the image to the D: partition. This image will be a large file or series of files that contains all the data of C:.
5) Now allow them to use their computers for about a week. This will allow them to move things around, customize things to their liking, etc.
6) Come in a week later, check for viruses, malware, etc. If it's clean, then create a SECOND image. This is your "broken in" image.
[6.1) If it's not clean, try to find out what caused the problem - go back to the original image you just made, and copy it back on. Show them what caused it, and tell them not to download whatever program caused it - and come back in another week (go back to step 5).]
7) Now burn your image files to DVDs - or stick them on your personal removable hard drive. Next time they have a computer issue, you can reformat/clean install/AND get back to their original settings/preferences/etc in about 15 minutes, by reimaging their computer. Unless they manage to royally screw things up, you don't even need to bring anything but a boot disk/cd, because their backup images should be on their D: drive. You have backups personally just in case too. ONE LAST THING: Before reimaging, temporarily backup their desktop folder and their Favorites folder in the D: drive. If you want to be really safe, backup the entire user directory in C:\Documents and Settings\. ALSO - do a quick search for documents that may have been saved on the C: drive. Then restore their desktop/favorites after reimaging.
8) After re-imaging, do windows updates.
I do this with my parents and friends the moment anyone needs a clean install. It adds about 30 min onto the process but then allows you to repeat it