If it's running Vista you need to, at a minimum, double the RAM, that machine came with 512mb. Vista likes 1gb or more. If it's running XP memory is not the issue, as 512mb is fine(depending on installed software). You will need to look at what is loading in the background. Background programs are not always a virus, ad-ware, or spy-ware, sometimes a legit program that you installed will cause problems. It could even be something as simple as not enough memory allocated to video.......
Bottom line, could be any one or more of several possibilities......