As some have mentioned both MSE (Microsoft Security Essentials) and AVG are both decent free anitvirus. I've used MSE for 3 or 4 years, and I haven't picked up a virus yet. Malware Bytes is also a good anti - malware program, so in combination with either MSE or AVG, you'll be pretty well protected. CCleaner from piriform is a decent piece of software for cleaning out your C drive, and correcting registers issues.
If your PC is freezing or slow just:
1, update your software.
2, turn off anything that runs upon launch, things that you don't need or use.
3, run a full scan, with both your antivirus or choice, and malware bytes.
4, defrag your hard drive.
5, run ccleaner to check for registry issues, and to clean your c drive.
6, (optional) use windows disk cleaner to delete any shadow copies or back-ups that you don't need.
EDIT: Someone made a good point. More than one antivirus will give you bother, as they'll conflict with each other.
1 antivirus + malware bytes + ccleaner = happy campers.
