1. The computer may be slow for additional dust-sand. The opening of the CPU, but at least once a month to clean dust.
2. The PC may be slow due to the virus. The regular virus scans.
3. C drive all over the place can be filled if the PC is slow. Keep unnecessary data on the C drive to another drive.
4. Install or uninstall the application, the PC is too slow to go slowly. So needless to refrain from any software installation.