^^
Sice his PC is HEAVILY (and i stress on that word) infected by Virus(es), then it may make it difficult for an Antivirus to get installed on his syste, coz the viruses modify the registry as well as prevent a smooth installation of an AV S/ware no matter how powerful the s/ware is, but still we need to install it first b4 scanning our system.. wich the virus will not allow
SOLUTION-1: Get connected to the NET (if the virus allows the connection that is) then try a KASPERSKY ONline scanner as said by Moto..
SOLUTION-2: IF that is not possible, (it wont be wise to backup ur data and u dont wanna format either), so just connect ur HDD with ur friend's PC (provided hs/she has a good AV installed prior to ur connecting of ur HDD) then u can scan the whole drives and TRY To solve ur problem,
BUT, Please install a good AV onto ur friend;'s PC and save him from getting his Drives infected too..
SOLUTION-3: IF that also fails, then FORMAT is the only option....
Cheers n e-peace....