I personally use the program, but my advice is that you shouldn't get into these sorts of programs if you don't know what you're doing. Mucking around with registry keys and such can have a negative impact if you choose to delete the wrong thing.
Now, SUMP: It's a decent program, but don't take the extra "registry fixes" it comes with. Just the no-bells-or-whistles package does fine. Carefully select what you want it to clean.
For the not-so-advanced user however, I'd recommend a safer, free alternative - Ccleaner.
http://www.piriform.com/ccleaner
Go to that link, and get the 1mb or so program. It's very useful, deletes temporary files and can safely remove obsolete registry information without worry.
But until you gain more knowledge of these things, I'd advise you to be cautious.
PS: Windows defragmenter is also good!