Exactly what Kelsey said. Quick scans only look at certain files that are most often targeted by malware and viruses, but tons still hide in other parts of your computer.
"Clearing up files" doesn't work very well either. Not only may they have edited something in the PC's registry, however they also could've likely edited a program that you use frequently and believe to be safe (until something infects it). There's endless possibilities in which something can get in your computer and wreck havoc. Malware and viruses are not the same thing, however they both need to be searched for to install maximum security.
Again,
highly recommend getting this:
https://www.malwarebytes.org It's safe, and works very well along side an antivirus. (This is an anti-malware program that searches for even the more recent types of malware including rougeware, spyware, etc)
https://www.malwarebytes.org/support/guides/mbam/Detection.html
