I’m told that even though one defrags and frees up disc space and/or deletes programs–info still remains in the "Registry" what the heck is that and how do I clean it out (when do you clean it?)

Windows Movie Maker came with my computer, now it doesn't work (error/close every time I open it.) This happened because I installed a program that was made before XP existed (and at the end it told me that it may cause an error but wouldn't let me cancel)
Anyway, I uninstalled that other program but I want Windows Movie Maker back, so what are the risks when I run the software repair wizard that will repair the programs that came with my computer?

(Does it try to repair ALL programs, or will it let me choose one?)
(I have all that stuff from when i first got the computer– stored in my D drive for recovery)
(Unexpected error, "has to close")

I'm not worried about viruses, I'm worried about further software errors
some people are saying I should download it… But I already have it! I don't understand downloading it…

Ive heard there are some 'Free' downloads of Registry cleaner on the web!…but everyone ive tried has a price?…not always apparent…what I need is a "FREEWARE" version!…any ideas?…Im in the UK…
I forgot to mention "for Vista"?…

I just formated me computer yesterday in an effort to get rid of the constant "Messanger Service" pop-ups from RegistryCleanerXP.

After the format, everything seemed fine, but today i was browsing the net and i got the same pop-up as before.

Why??

I have only gone to two or three websites since the format, and everything seemed fine. But now the pop-up's are back.

How can i get ride off them premanatly?? And how are they getting one my computer??

Any help would be GREATLY appreciated.

Thanks
Thanks for the info Gary,

But i was just wondering, if doing it through services.msc will affect anything else on my computer??

I had actually read that method of fixing the problem, but some people say that if you do it through services.msc, then it will disable other things on your computer such as MSN messenger.

Is that true?

On my old computer (an emachine), I had an icon on my desktop called “Disk Cleaner” where I can delete files on my computer but I don’t have disk cleaner on my new computer (a Dell). Is there another program on my new computer that is similar to disk cleaner so I can delete old files and stuff and clean my computer?