There is no one reason why a computer would do this. I would start with general maintenance solutions.
Download CCleaner and run it to remove all temporary files. If you don't want to install anything, you could instead download TFC - Temporary File Cleaner. It runs without having to install anything.
Defrag the hard drive. Go to Computer, right-click your hard drive and select Properties. Go to the Tools tab and select "Defrag Now". Select Analyze first. If it is more than 5% fragmented, defrag it.
It could be caused by a virus. Download, update, and run Malwarebytes Anti-malware. If you don't have an anti-virus with real-time protection installed, consider installing Microsoft Security Essentials as well. It uses very little resorces.