It shouldn't hurt your system to do the thorough scan as suggested so I would do that first. If it passes then go to Start|Run and eneter this command:
devmgmt.msc
When Device Manager comes up, click View\Show Hidden Devices.
If there are any devices with corrupt drivers you will see a yellow or red mark. Right them down and post them back.
Also in Start|Run you could enter this defrag c: -b and see if that has any effect on speed. You could also do security scans of av and malware but later. Hopefully someone can jump in with a more decisive answer. Tomorrow.