For once, your being skeptical is a good thing. Too many programs promise silly things lie "make your computer go faster". But it's BS. Lots of that software is very dangerous.
Sometimes good freeware exists.
- CCleaner is an excellent registry cleaner.
https://www.piriform.com/ccleaner (download free version from FileHippo)
- Assuming Windows, Microsoft's own Security Essentials is good.
http://windows.microsoft.com/en-us/w...tials-download
Sometimes payware is better.
Kaspersky is excellent, as is
MalwareBytes. But again, those are payware, not freeware.