Which Free Antivirus Program Should I Get ?

I just built my pc and everything has been a success!

I attempted to install McAfee VirusScan Home Edition 13 on my new PC since I got it for free from school.

Before installing a message popped up saying "This program has known compatibility issues" and at the bottom it says to check for solutions or run the program. I ignored it and ran the program anyway. After going through the steps installing the security center, virus scan, firewall, siteadvisor, and etc. a message pops up after each item has a check next to it.

The message says McAfee integrated security platform installer has stopped working. then i have to close the program. I believe this is because it is an older version of McAfee that is not supported by Windows 7.

What is a good antivirus software that I can get for free?

I heard that Avast and Avira are good, but i am not sure which one to install.

I am not too sure about Microsoft Security Essentials because that seems to be a prime target to try and hack.

Any suggestions?