Has anybody been using Clam Antivirus for an extended period of time?

I was wondering if it performed comparably to comercial antivirus products in a Win XP environment.

Also has anybody been using it on Win98? Yes I know linux is better but try telling that to a 75 year old woman who's son is a windows zealot. I am trying to get her good antivirus protection without having to shell out a lot of cash. If I get her using open source in Windows I can wean her on to Linux later!