C drive keeps filling up on its own

It doesn't hurt to periodically go to Accessories\System Tools\Disk Cleanup. When it tells you how much space it can regain, check the boxes for Temporary Files and the Recycle Bin. After it runs, reboot. As far as I know, you don't need to be an administrator to do this.

I would also run a few more antivirus programs if Malwarebytes chameleon found 167 viruses. You could try a rescue disk which runs from boot. Kaspersky do one.

I picked up one from another pc via a usb stick recently - my av caught it but when I googled it, it was a worm which replicates and fills up unused hard drive space so you may still have something in there.