If you can wait for the fall, Microsoft's new antivirus has gotten really good review from beta testers.
The actual security aspect aside, the most important part of security software is undoubtedly performance. Since MSE doesn't include many of the features of OneCare, this is an area that Microsoft has a chance to excel in. In fact, the company includes three features in MSE to keep it light: CPU throttling (the system will remain responsive to the user's tasks), idle-time scanning (scans and updates use a low-priority thread and only run when the PC is idle), as well as smart caching and active memory swapping (virus signatures not in use are not loaded into memory).
It should also be noted that MSE is very small; when MSE first leaked out yesterday, we noted that the installer sizes range from just over 3MB to just over 7MB (the folder installed takes up about 11 MB). The leanness of MSE is also evident when looking at the system requirements:
For Windows XP, a PC with a CPU with clock speed of at least 500MHz and at least 256MB of RAM
For Windows Vista and Windows 7, a PC with a CPU with clock speed of at least 1.0GHz and at least 1GB of RAM
VGA (display): 800x600 or higher
Storage: 140MB of available hard-disk space
An Internet connection is required for installation and to download the latest virus and spyware definitions.
Microsoft announces free antivirus, limited public beta - Ars Technica