I doubled the RAM, but it's just as slow?

My media centre PC (only connects to TV for streaming/movies, etc) has been slow so it seems like it'll never boot let alone run difficult stuff. With only 1GB Ram, I decided to double up to 2GB - but now its done, the pace is unchanged? I've done this before and it has always worked really well, so I'm confused. Dont laugh, but it runs a Duron12000 - i know thats old, but it used to work better before i created the media centre? XP Pro installed, on a sata drive with a new installation recently. I can get a new CPU pretty cheap, so will try that next if no-one can think of anything else to do?