It won't cure all of your Mac's ills, but a quick and easy first step toward bringing a sluggish Mac back to health is using OS X's Disk Utility to repair disk permissions.
Sometimes, repairing permissions will fix stubborn printing issues, problems opening a file, or even the ability to log into the computer as a user (as you).
Here is how to Repair Permissions:
Open Disk Utility in Applications > Utilites
Select the name of your startup drive in the left column. Make First Aid is selected in the right area. Then click on Repair Disk Permissions.