Hmm, that's a pretty old rig you've got there. I'm guessing you're right at the bottom end of the recommended specs?
In general, though, the best ways to squeeze the best performance out of a rig is...
1. Make sure your drivers are up to date
2. Kill any unnecessary processes. Anti-virus software is often a performance killer, but since you're connected to the net whilst playing, you probably shouldn't turn it off. Changing AV software might help though - I've heard AVG hogs far fewer resources than Norton, for example. But make sure you turn off email, messenger etc whilst playing games.
3. Defrag your harddisk - especially make sure the page file isn't fragmented.
4. If you haven't reinstalled Windows in a while, do that. Your registry inevitably gets bloated after years of installing and uninstalling stuff. A nice clean system will usually boot faster and you'll see an improvement in performance. Maybe also take the opportunity to do a ghost image of the clean install to save time next time.
5. Start saving up for a new rig and treat yourself