Try Microsoft Security Essentials. It's free, it's incredibly lightweight in terms of memory and CPU (just checked - 2.4 MB for the process, and 0.0% CPU), and seamless to install and run.
I've read mixed reviews of the Essentials. Some recommend it, but most seem to feel it is the barest minimum, intended only to sweep up the home-computer laggards who would not otherwise run any A/V, even the free third party ones.
It's surely better than nothing.
I ran OneCare for two years. It was too damn big, but it sure did a job. It was also abusive of the CPU at times.
Tell ya what, I'll give Essentials a try and let y'all know how I like it. Nothing to lose.