So disable the virus protection when you work with graphics...
And set your system up to boot up as Windows Networks and then pw protect it... if he gets around that... pw protect the boot up at the CMOS level... Orrrr you can tell him to quit stealing or you will turn his thieving tail in personally!
Or tell the deadbeat to get his OWN computer aand connection... preferaably in some OTHER house.
I'm on a roll already with one cuppa in!