Avast is the best!
I have a 7 1/2 yr-old Windows XP desktop with only 256 mb ram. I had McAfee (full coverage) FREE w/ my ISP subscrition. Dumped it bc it was so greedy of time and space & resources, & was SSOOOOOO SLOOOOOOW. After that I tried various other AVs, most notably Norton & AVG, but after a lot of time and searching have settled permanently on Avast. You'll find Avast has the lightest touch of all the AVs out there. It does a thorough job, is very fast even in full scan mode, is easy on ram, and best of all, NO BLOAT! It puts up a solid door on your computer & won't let anyone in w/o proper ID and an invitation, and does fast scans using common sense and courtesy for you, the user! And NO, I do not work for Avast, nor does anyone in my family or anyone that I know. This is an honest appraisal from a retired 67 yr-old not-quite-a-computer-newbie-anymore.