Buy a new computer from the sounds of it, anything you could upgrade to really wouldn't be worth while as you can buy a better computer for not much more (and possibly less depending on the parts you would need)
2 gig of RAM will speed up the computer, as will a new Windows installation. Aside from that, if that is not fast enough, need a new system. You may be able to get a 3 or 3.2 gig CPU for your computer, that should help a lot also.