Antivirus programs have always been my main cause of sluggish computer performance. In the last year, I've souped-up four computers by adding a gigabyte of RAM. That took care of the sluggishness quite well, and made me feel like I had a new computer for fifty bucks or less.
I added RAM to a couple computers a few years back, and didn't really notice any change in performance. Sure did the trick this time, though.

-Joe-