CCleaner is one of them. Here’s a link of reviews of recommended registry cleaners, you may want to check it out:

Use the tool:

Here are three for you:
CCleaner:

and

Eusing Free Registry Cleaner (this one is the best):

Abexo Free Registry Cleaner:

Also as for the 600 problems found, it does that to scare you into buying it, your computer doesn’t really have that many, I have seen ones where it told me I had 1000+ errors, I then did a scan with an excellent registry cleaner I bought and it came up with 11.
Don’t use free trial versions as a lot of the results are made up, use one of the above or purchase if you want.

I know this isn’t free but it’s the best cleaner of all time. Try winsweep.. It does wonders on my computer.

Yes, Ccleaner is ok, but Eusing does a better job and is also free:
More details:

Also download Malwarebytes Anti-Malware Removes Antivirus 2008/2009/XPAntivirus and many more:.
Download, install, update and select Quick Scan.
Remove all infections that it finds after scan.

Spybot S&D:
Download, install, update and Immunize, then click ‘Check for problems’ then when complete select all and then ‘Fix Checked’

“Yes” ccleaner. (http://www.ccleaner.com) and it is free. It is best and safe to download this program from “CNET download” I gave you a link above but when I click on it it took me to where I have to pick and choose, and there are so many companys that offer this free download, but some are not safe and that is why I recomend that you go to cnet downloads to download this program as this download site is safe. Good Luck