Forget Registry Cleaners and other Optimizers. They can do more damage than good. The only cleaner we would recommend is CCleaner from piriform.com and only use that with it's default settings to clean out temporary files and temporary internet files. It has a registry cleaner, but it is not necessary to use that.
As to free anti-virus scanners, most of the big named Anti-Virus utilities have free options and they all work, so asking which is best is difficult to answer. You will get as many different answers as replies. We suggest using an AV and also a couple of stand-alone manual anti-malware scanners that do not run in the background, like MalwareByte's Anti-malware and SUPERAntispyware.
What are you using now, and why do you want to change?
I know i am cheep, but I also know that there are good free antivirus and computer cleaners out there. I have windows XP. I have tried a few and wanted to know if anyone had advise before I change or try something different?