What operating system you running? If it's a Windows based system and is not XP or 2000/NT Server and has gone a long time without being reformatted and reinstalled, it could be your OS. A lot of people seem to experience deteriorating performance as time passes with Windows.
Is the internals clean? I have to clean out the inside of my parents machine quite regular as they don't have a clue. The amount of dust in the intake fans and heatsinks defy belief. I can drop temps by a good 5C every visit