You could start by cleaning things up on a software level. Disable unwanted startup programs in MSCONFIG. Run CCleaner and get rid of all the built up rubbish, defragment your drive etc.
Once you've got your OS sorted out, you can determine whether you actually need an upgrade.
X2, Worship. And also, if you want vast improvement (for instance if you really don't need half the crap that ever got put on your computer) you could just reformat and reinstall and start fresh. It's amazing how speedy an older computer can be on a fresh install.