Just as the title says,....and I stress the word 'free' because not only is free my favorite price but it's also the only price I can afford :-\
I know I've got registry errors and a lot of un-needed registry entries etc, it slows down my comp on start up and I'd like to do something about it.

I am using System Mechanic for several years already. This program gives me complex maintenace of my computer. Although it isnt free, when you buy a stand alone version (now 7.5) you can use it for any time you want. I am very satisfied with it. It isnt ideal but I dont know anything better for now. I think its worthy of some small investment.  :)

http://www.iolo.com/sm/7/std/

There are also some another free programs here but I dont have any experiences with them. There are some links here for the download.

http://www.ccleaner.com/

http://www.wisecleaner.com/

http://www.eusing.com/free_registry_cleaner/registry_cleaner.htm

http://www.download3k.com/System-Utilities/Registry-Shell-Utilities/Download-MISPBO-Registry-Cleaner.html

I use Regclean myself. It's simple and free. Though not necessarily the most thorough one.

Also go to download.com when I really need one, and you can download tons of different types of free programs and free trials of paid ones, even. They also have editor reviews from the site and customer reviews, so you can check up on what other users think.

I've used ccCleaner with good results.

and by that I meant that my computer still works.

However I was looking through the registry yesterday for a specific key
and found lots of stuff that shouldn't be there. So it didn't really clean it very well.

There's another part of the program that removes unecessary files and whatnot like prefetch data and .tmp files and that works well.

http://www.ourfreestuff.co.uk/cat/Home/CD44/goog_cont_bm_freebies

Here be an URL Apep for ye to some free stuff but if ye can get a good Computer Mag, they usualy give a load of stuff away.    I gor stacks of reg cleaners all free Apap too big to send by mail but I will put them all on a disk for ye if'nm ye wants lad!

It is wise to use more than one cleaner of ANY type so de mags say lads.  ;D  ;D