Now this is on the basis that I understand it:-
When XP want to do something, like run a programme file, there are bits of that file's requirements all over the hard drive. All the adresses (and other stuff) of those bits are located in what's called "the registry". The snag with it is, that when you want to change or (God forbid), remove something, it's a bit tricky because all these bits are spread about.
The 'removal wizard' does it's best but does NOT clean the Registry on exit, so after a while, the Registry is full of pointers to non-existent files or information as well as other bits of loose stuff.
Naturally, Windows takes time to look up something every time the programme is run and a lot of useless entries just wastes search time.
A Registry cleaner simply does all that - and closes off the open entries.
Naturally, this is not the same as anti-virus software. . . . . . . .
DT_Xtremez_28:
PS. Thanks for the suggestions, guys. I now have Cc