you are on a looser here ... you can't scan one computer from another (not with home software anyhow)
you don't say if a router or any other kit is involved .... so the howto could be achieved in a couple of ways .....
one of your problems is ... if you have a virus ... your visus protection obviously isn't up to the job ... so there is no point scanning using that.
you can either ... use a bootable CD that some virus prot sellers provide.
try downloading AVIRA, AVG or one of the freebies and cross your fingers
actually most viruses these days are infact spyware ... so you could well get away with it
or.... you can pay for one of the biggies....
or crash and burn the machine