I'd suggest it depends on what you want to run it in.
For a low powered computer such as a Netbook, I'd suggest Avira AntiVir; for a powerful Windows 7 PC, there is Microsoft Security Essentials. Otherwise, there are Avast and AVG, of which I prefer Avast, although it can cause a mediocre PC to slow down quite noticeably.
If you deal with Barclays,the full Kaspersky Commercial Edition is free for 12 months.
I relied on MSE for twelve months and then it let a load of nasties in a week ago which I am still trying to get rid of.