I have 3 different Registry Cleaners (Registry Mechanic 6, System Mechanic 4 and Registry Cleaner) and each one of them found different problems with my registry. Which one should I believe?

I have had my computer for about 3 years and it is becoming slow because of registry errors and things of that nature. Does anyone have a good suggestion for a program that can help and clean up computer? I have a dell and I on XP. Thanks in advance.

Any new software that I try to download gives me an error reading, "The Windows Installer could not be accessed, this can occur if you are running Windows in safe mode. Contact your service provider". Please help! I am trying to upgrade my iTunes software so I can sync my iPod to my computer but Windows Installer is preventing this for now. Thanks.

I downloaded what I thought was a completely free registry cleaner last night… turns out it was a trial. I don't have any money to spend, and I have enough crap wrong with my computer already, so I'm looking for a FREE and TRUSTWORTHY registry cleaner. Any *good* free anti-spyware/malware/virus programs are appreciated as well.

I'm looking for a registry cleaner that will find the errors and fix them for free. Not none that will scan my computer tell me how many errors i have and then charge me to fix them is there a totally free one that dose everything ? Thank you in advance for your help.