You guys are confusing viruses with Spyware and Browser Hijackers. These are not viruses. Anti-Virus software will not protect you from these. You need both Anti-Virus AND Anti-Spyware software to defend your system.
As for Avast! and Microsoft Security Essentials, both of these are great Anti-Virus programs. Microsoft Security Essentials also, (although not very well known), integrates itself into Windows Defender to use its features to protect against Spyware too. (Contrary to popular belief, Windows Defender is only turned off, not disabled, when using Microsoft Security Essentials, as the program's GUI hooks into Windows Defender's engine itself).
I have also heard good things about MalwareBytes over the years, but I have never used it personally.