I have seven (7) total choices which are all free, synergistic and effective from my experience.

1. AVAST antivirus — It automatically updates its database and constantly runs without slowing down your system. A good first-line choice.
2. a-Squared — Also Free and runs a few great scans which can detect and remove spyware. Smart Scan mode also includes Windows folders without going “too deep”.

3. Malwarebytes’ — An anti-malware program which is really good. Removes trojans amazingly fast.

4.  CCleaner — Crap Cleaner removes crap. Takes a little more attention to use, just to make sure you don’t select things you want to keep, and speeds up your machine noticeably.

5. Free Windows Registry Repair — From Regsofts. Performs a basic scan and removes dead keys and creates backups when doing it.

6. SUPERantiSpyware — A nice program which has quick scans and is easily updated, helps get rid of tracking cookies without hassle.

7. HijackThis — A very good, more advanced tool for killing processes and deleting files on startup. A must for those with hard to find infections.