I have 1 GB RAM (maxed). It came with XP Home Media 2004 and I upgraded it to 2005. Before I installed Vista Home Premium recently it was a little sluggish. After Vista, it has been a nightmare, looking for drivers, etc.
Now my PC is as slow as one 15 years old. It takes up to 10 minutes to boot. When I open most programs there is a delay after I click, then an empty box comes up and slowly paints itself. However, once the program, e.g., Word finally comes up, it seems pretty normal.
I have kept at least 40 MB free space and I defrag and virus check regularly.