Do not waste your money having it looked at. They won't do anything you couldn't do yourself.
Scan for viruses with a decent checker.
Scan for spyware and malware.
Uninstall stuff you don't use.
Check your startup items and remove stuff you don't use or need.
Check your registry structure.
Defrag.
Remove excess icons from the desktop and bar.
CCleaner is a good place to start
http://www.ccleaner.com/
What Antivirus and spyware software are you using? If you use any resource heavy security centres like Norton or McAfee then look at lightweight alternatives.
If you have the relevant CD's then a fresh install is a guaranteed way to speed things up as all the crap that got installed over the months/years won't be there anymore.