First of all, remove all programs you don't use.
Are you running XP or Vista? If you are using XP, I know of a guide you can run through to remove unwanted start up items.
If your still not happy, a defrag of your hard drive can completly clean your system out, giving you a welcome boost, though you need to back up everything.
Then theres overclocking, but overclocking CPU's can be a tricky process, and I believe you have a Dell? Someone may be able to help you with that if you really wanted to try it.