A new HD/New install is what i usually do if an older install of windows goes wonky. Grab the data off the old HD, back it up on the new/burn to media; nuke old drive, new install on that in a diff PC (or nuke the old drive, and keep it in as a 2nd/3rd/4th HD data dump).
In the 2 main computers on my network; one has 4 drives on it, 200gb data, 80gb system, 60gb data, 80gb external; the other has 2. 120gb system, 200gb data