Why is my computer running so slow, it used to be much faster?

There can be many causes for a general slowdown of your computer. Some are not necessarily security or virus related. You should check all these items to be sure.

1. Run an updated virus scan.
Or Scan for Viruses online.
Sometimes it is a good idea to run more than one to ensure nothing is missed:

Symantec Security Check

2. Download some anti-spyware program(s)

3. Perform Windows Memory Diagnostic

4. If you have file sharing software such as Kazaa, Bitcomet, Morpheus, disable it.

5. Run Defrag.
Start/All Programs/Accessories/System Tools/Disk Defragmenter