So noramally I just always keep my imac running and on and its always really fast! but i had to turn it off when i was going away for 2 days and its ben slow ever since! what do i do?! how do i speed it bak up?!

Dig the malware out of it that macs don't get.

You don't have any malware slowing it down. Ignore the sarcasm of Dr. Dave. He hates Macs and makes up stuff about them since he knows nothing about them

You should tell us more, such as....
-- Which Mac (model and processor speed)?
-- Which version of OS?
-- What task is slow? Startup time? Shut down time? Internet page loading? Flash video loading? Applying filters in Photoshop?

With so little info, no one can make much of a guess as to the problem. Until you can submit a meaningful question, try this:
-- Open Disk Utility and verify disk. If problems found, boot to the OS X DVD, open Disk Utility, and repair disk.
-- Restart holding the shift key for 30 seconds to get "Safe Boot" in red letters in the center of the screen. Log in, and test the speed.
-- Use Software Update under the Apple menu to get all the available updates, install all of them, restart, and use Software Update again.