Microsoft Security Essentials (MSE) is becoming a good alternative for those who prefer to have a free security solution.
How to get the Latest MSE Definitions
If your system cannot connect to the internet, due to a virus infection, this is your only way to be updated. If you are experiencing problems updating your antivirus you can use this method.
The following links download the utilities, different for 32 and 64 bit operating systems, that install the latest definitions:
Definition updates for Microsoft Security Essentials on computers running x64- based versions of Windows
Definition updates for Microsoft Security Essentials on computers running x32- based versions of Windows
Hint: Am I running Windows 32 or 64-bit?
When installing on Windows 7 and Vista you need to run as administrator. Right click mpam-fe.exe, click Run as administrator, and then click Run.