1. which scan is this
Both scheduled and custom scan can exclude folders and files you do not want scanned ie exclude a folder where you have large avis or zip files. these files slow the scan down.
In scan setting look for option ticked "use minimum resources" unselect this.
Try a custom scan excluding zip files and try it with Program and documents only to see what speed it takes.
Have you a folder in windows called checksur? It is made by windows update when an update stumbles and the files it contains can slow things down.
Exclude network and external drives
That said have you done a chkdsk c: /r at the command prompt. It will require a reboot and check your disk health.