Before answering: you've been warned in another thread about multiple posting - use the edit button to edit your earlier posts, not make multiple new posts. It's against the forum rules to multi-post because it's akin to spamming.
Short answer: there is no "best" free AV program.
Long answer: Most people use a combination of AV programs because each AV program has different levels of effectiveness. Get MS Security Essentials and find a combination of anti-Malware/Adware/Virus programs and a good Active Protection program. Naturally, if you pay for a good AV, you'll get most of that in one neat package. AVG is the most popular free AV program, not because it's any good but because it's free, easy to get and easy to use. It's also the easiest to infect with specific voracious virii, but you shouldn't be encountering such things if you stay out of illegal sites.
It's also worth mentioning that any good AV program will make a noticeable impact in system performance if your computer is a lil bit old/low spec, but since you're asking I don't think that matters to you. You really should consider getting a commercial AV program like Kaspersky. You can get a year of it for £15, or a year with 3 users/computers for £20-25, depending on where you look. That's less than the price of a game, so I think it's worth it, especially since I just happen to have 3 computers that need constant protection (whilst I can clean the others out myself, not that they get infected in the first place).