Forget the so-called "free" anti-virus software. Rather spend a few bucks and get some decent anti-virus software, like NOD32. The following anti-virus software sucks big time:
AVG
Avast
Panda
Symantec / Norton Anti-Virus
McAfee
NOD32 have obtained over 60 Virus Bulletin Awards for their software. I have not used Microsoft Security Essentials yet and therefore cannot comment on it. But always remember that if you go with one of the above packages, you get what you pay for and must not be surprised if you have to spend regular time to reformat your pc because of a virus / rootkit / trojan horse.
Malwarebytes Anti-Malware is excellent software but sometimes infected pcs refuse to run it. So you may try the following utilities:
Combofix -
www.combofix.org
TDSSKiller -
http://support.kaspersky.com/downloads/utils/tdsskiller.zip
The TDSS family of rootkits also prevent anti-virus software as well as some utilities to function properly. When infected with a TDSS family rootkit, you may notice when executing software to disinfect, that it creates a fake process consuming between 1900KB to 1910KB. The TDSSKiller utility will take care of that :twisted:
Through experience, I've also noticed that some online virus scanning websites fails to disinfect a pc properly - rather use Eset's online scanner:
http://www.eset.com/online-scanner
At the end of the day it all comes down to whether you have decent anti-virus software installed or not.