mac running slow.

10.5.1? Provide an updated profile and the machine model. For general maintenence, see

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X,
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X.

Do you mean that it's slow to load web pages, or that everything is slow? If you're running OS X 10.8 or later, please update your profile.