In my systen Kaspersky use 55 Mb ram....Avira is lightest based on my tests, using between 50-60MB at idle. Kaspersky is next, using 70-100 at idle. Bitdefender as usual uses almost 200MB at idle (as much as McAfee!). As far as subjective "lightness" tests, I sense the least slow down with Avira, followed by Bitdefender, with Kaspersky feeling the most sluggish (which is not bad compared to some others). Personally I won't use Bitdefender products (or Mcafee) simply because they use so much private memory at idle. I'd go with Avira Pro in your case.