Oh, man, I've used ERUNT to save a couple of systems! That program DELIVERS!!! It's one thing to use it for backups?quite another to find you really NEED it. First time was an emergency, and I could not get into Windows at all. I had to use a DOS disk. It was scary, and I had to read what to do, because of the sense of emergency. But in a few minutes I had that system right back to where it had been. I rate the combination ERUNT and NTRegOpt as two of the most brilliant freeware programs on the internet. Here they are: http://www.larshederer.homepage.t-online.de/erunt/
? However, NTRegOpt will lose the ability, after awhile, to optimize the second hive of your registry if you are using XP 64. You can still use it, but not for that hive. And in Vista, you will not be able to modify your registry unless you do your restart with the User Control turned off. (Controversial, but I keep it off, myself.) That's here: http://www.petri.co.il/disable_uac_in_windows_vista.htm
Back to CCleaner and other registry cleaners: the discussion of these cleaners is very heated. A registry cleaner can kill your boot-up ability. That leaves quite an impression on people. Myself, I like registry cleaners. Other folks hate them. I still use RegSeeker in its very most dangerous fashion in 32-bit XPs, although the same RegSeeker will wipe out XP 64-bit. It will work in x64, but it will wipe out x64.
CCleaner can use its huge popularity to maintain a very good, realistic database. CCleaner caused me immense, long-lasting grief when I used to use Word 2000, because there was a seemingly unimportant log file that CCleaner would change in the registry. That did no harm in itself. But Word 2000 had a safety check that would see the change, and would then restore a default Normal.dot system, just in case the change represented unauthorized tinkering. The result was that Word 2000 would lose its settings. To this day, I uncheck CCleaner's box for cleaning Office files, although with the advent of Word 2003, a person could restore Office registry settings anyway, with a tool.
I suppose I've used CCleaner, oh, maybe 3,000 times? Hard to say. It could be more. That Word glitch was the only one I ever encountered.
Oh, I still keep the 2004 System Security Suite around. Makes for a nice restart, cleaning off the files as it does the restart. CCleaner holds onto the files until the restart; SSS actually causes the restart to happen. Apart from that, CCleaner eclipses SSS, so you don't need SSS. It's redundant, and not as sophisticated as CCleaner. But it's here anyway: http://igorshpak.net/