Very quick question about using an antivirus on Windows XP Service Pac 3...

On my own old Windows XP service pack 3, I need an antivirus light and effective, although I use both Sandboxie and AppGuard, I'd like to have still an antivirus just in case.
Here are the criterions:
I have 895 MB of ram memory-which is a very limiting factor for antiviruses like Avira since you need to have at least 512 mb of ram memory free, I want something lighter, but yet at least effective.
Is there any antivirus that could meet my needs for both security and usability and compatibility?

Avast and Eset Smart security nod 32 antivirus both use 128 mb of ram memory but I'm not sure how light they are on old Windows XP service pack 3 systems, any ide?
Any help is welcome.
Big thanks in advance.