Computers can run slow for many reasons but before looking for hardware causes or reinstalling your computer from scratch you might want to run through these steps first to eliminate some of the common causes of slow performance.
Check for and eliminate any malware (Spyware, virus', trojans, rootkits...) download and scan your system with a quality spyware scanner such as malwarebytes, hijackthis, spybot, Ad-Aware) AND a quality virus scanner (they are NOT the same thing).
Anti virus
http://www.kaspersky.com
http://www.avira.com
http://www.avg.com
http://www.malwarebytes.org/
anti spyware
http://www.kaspersky.com
http://www.superantispyware.com/
http://www.malwarebytes.org/
http://www.hijackthis.de/
run MSCONFIG (from the run line) and check the "startup" tab, Remove the check box from programs you don't need running on startup.
Run "Disk Cleanup" (from "start/programs/accessories/system tools/disk cleanup" and let it remove all the unnecessary junk that tends to pile up over time. A more thorough tool for cleaning disk clutter is “ccleaner” which you can download here - http://www.ccleaner.com/
Run a Disk Diagnostics (from "start/programs/accessories/system tools/disk Defragmenter"
Run a Disk Defrag (from "start/programs/accessories/system tools/disk Defragmenter" and let it defragment your drives.