You know the drill - corrupt OS, Virus, screwed up registry, etc. The solution usually boils down to reformat and reinstall, unless ... You regularly image your OS install. Use something like Acronis (costs $50), though there are "alternative" ways of getting it. Schedule it to image (I usually do once a week, and keep up to four images covering a month), invest in another hard drive or usb drive (these are dirt cheap now), and should the situation arise, you'll be back up and running in less than an hour. These are essential if you like to update your hardware regularly like myself, but at the very least, makes the whole format and reinstall ordeal much easier to deal with.
I picked up my copy of Acronis Home 11 (I think, whatever the latest one is before the recent new release) for like $15. Good package, but installing it on your computer installs a bunch of services. I prefer using the CD to boot. For Vista, imaging usually screws up the activation code, so you'd have to go call Microsoft and go through their automated process of reinstalling it. I've done it three times now when swapping out OS HDDs, the first two times I got some Indian dude, and the third was all automated. The first time was early on in Vista's life, the second time the guy didn't bother with too many questions, and the only question the automated machine asks is if you're using your license on multiple computers and for the activation code on your screen.
That'd be assuming I didn't use some alternate way of activating Vista... Even if you are a legal owner of the software the very issue EF2 just mentioned is worth it alone to crack the software, IMO anyhow. In any case, yes Imaging kicks ass, I have a spare 80GB that I keep the base install of Vista backed up to, I've been using Acronis as well.
Norton Ghost does this too if anyone is interested. You can back up entire partitions, hard drives, etc. to a second hard drive or an image file.
I use to use BartPE with Norton Ghost. No need to install software. Bootup with BartPE and run the Norton Ghost utility to create an image or load an image. Now, I just do slipstream installs. Hardware changes and bam! An old image is worthless. Slipstream a dvd with your favorite programs and I'm set to go. I might need to install a game or two, but no big deal.
What version of Ghost reads SATA drives and partitions? My copy of 10 sucks ass, which is why I went with Acronis TI11.