There is nothing better than buying a new PC. Also, please tell us what 'scannow' is which you performed before contacting live support? SFC scan?
Based on whatever live support said it seems like you contacted fake support. Or maybe it was genuine MS support but I would not give anything to live supports. If Malwarebytes and other scanners are showing clean results then I would worry that my PC is infected.
To remove remnants of other viruses, if present, I will scan my PC with HitmanPro, you should scan with it also. HitmanPro scan will remove remnants also then I would consider my PC clean. Regarding system corruptions caused by viruses, most of them are fixed by scanners. If something is there then SFC scan can do the trick
and there is nothing better than a reinstall of Windows to fix system corruptions.
At this point, I would recommend you to scan with HitmanPro. Then run a SFC scan:
[Steps for Windows Vista, 7, 8]
Go to Start and search for 'Command Prompt', right click on it and click on 'Run as Administrator'. Then type this code and hit enter:
sfc /scannow
[Steps for XP]
Press Win key + R. Type 'cmd' and hit enter. Type "sfc /scannow" and hit enter.
After running SFC scan, if it find any corruption and was not able to fix it then consider reinstalling Windows (if you don't know how to do it then contact computer repair shop for that).