How long have you had it? Is it time for a new one?

If it's speed on the internet - probably the problem is your ISP or your router (or modem).

If it is processor-hungry applications like video or multi-track music - - new computer with more crunch (but you could try turning off all other applications to see how much difference it makes.

If it's general operational stuff have you defragged and emptied cache etc?

Have you thought about backing your data up and formatting C and doing a clean re-install (preferably with pretty much only OS on C and other stuff elsewhere)?