system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - What REALLY causes computers to slow down? Can it be stopped / reversed?

My computer is getting slower and slower with age and I don't really know why. I don't have any viruses, I have tons of free harddrive space, and plenty of RAM. Can anyone explain? 

EDIT: Thanks everyone for the sound advice. Lots of research to do. Finding some answers should be much easier now.  :)
gpt
I think your most likely cause is software.  OVer the years/months/whatever you will have installed a lot of software on your computer.  A lot of this software will have components that run in the background, this may be task bar icons or services that are idling on your computer waiting for a trigger (a disk to be inserted etc).  Installed software can also make changes to windows system files and registry which can also cause your PC to slow down.  

Windows 7 is a lot better than previous windows versions at coping with this due to UAC and a good memory manager but it will still be affected.  

To avoid this happening your best bet is to make sure you only install software from reliable vendors and you also only ever install software you actually need (a lot of people are big fans of installing heaps of stuff they dont actually use of need).

To fix it, you can do a complete system re-install using the windows install disks.  This isn't as scary as it sounds, just make sure all your files are backed up and then reinstall using your windows disks.  Note: this will also delete all your programs so be ready to redownload and install them.  After doing this, your computer will seem like new again!