PC slowdowns over time are often the result of having too many programs running at start up. A lot of programs are configured to do this. Where you don't actually need something to run every time you turn the computer on, it should be configured to not start with Windows.
This does not apply to security programs, it is important that they do start with Windows.
How long since you've done a disk defrag?
If you look in control panel>system, what are your system specs (RAM and CPU, and Windows version/SP?)
If you right-click each drive present and select "properties", what is the percentage of disk space used?