MSE is the lightest.. but scanning speed is slow.

http://www.pcmag.com/article2/0,2817,2353386,00.asp

However I don't trust MSE in my computer. You are using the same OS, software and AV from same developer and researcher. So what they missed is not covered in AV which suppose to be the last line of defense. I would only use MSE for an older computer and system requirement is an issue.

Nono its Security Tool da best...


But seriusly, you could try MSE as it is the lightest, I have a 256Mb machine and even Avira bog down that machine, mind you that Avira is one of the lightest around. If even MSE bog down the system , then remove that and try a HIPS only software such as Gesswall or DriveSentry . HIPS only software is very light but unlike AV you have to use your own decision to decide what is a virus or not, e.g if you download a movie file and HIPS tell you it try to modify startup, it is surely a virus.