It depends on a) what you mean by "slow", and b) what's causing it to slow down.
First off, do you mean a) all applications are running slow, b) certain apps/games are running slow, c) certain games are lagging while playing single-player/multiplayer, d) the PC is taking much longer to boot than it used to, e) the PC is taking much longer to access files than it used to, f) combination of 2 or more of the above, g) all of the above, or h) none of the above.
The above symptons could be caused by one or more of the following:
-- system page file has been set to the wrong size, causing the system to access the hard drive more than it should be
-- problem with hibernation file settings
-- temp files are accumulating on the PC, & need cleaned up (including having too large of a cache file in your browsers)
-- too many useless apps are set to start up when Windows starts (like the "accelerator" for Adobe Reader)
-- too many tabs open in a browser
-- multiplayer map servers have a bad connection, giving you a high ping when connecting to them
-- your ISP has an issue with its equipment, giving you a high ping/latency
-- your modem/router needs reset or replaced, as it's causing problems with your connection
-- you have adware/malware/viruses on your PC, causing it to slow down
-- you're attempting to play newer games at high settings with CPU/GPU hardware that just isn't going to cut it
Without more details, we can't really diagnose where the most likely problems would be, let alone the diagnostic & repair procedures to isolate & fix them.
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