Remove any programs that run in the background that you don't need is the first step. Another thing you could do is switch the theme to the classic windows 2000 theme and disable any fancy/special effects. Whilst this doesn't improve performance too much it gives the appearance that it's running fast and believe it or not, it makes a difference! I've done it on my dads laptop which is what I am using, also pretty crappy and it's made a difference.

If it's for gaming, tell us what graphics adapter you have, perhaps you could do a bit of tweaking if you're up to it... Plus for some games you can maybe download a config.

And like the previous poster said, tell us the rest of your specs!