monitor not displaying pc

my sons pc was working fine then last night it wouldnt let him open any programs or connect to internett just kept getting a box saying av antivirus activate now shut pc down and restarted and it seemed ok did an avg scan found nothing then this afternoon it wouldnt let him connect to internett but he could use msn for chatting so decided to format it so unplugged it all and pluged it into my monitor in my room pc powered up fans working and pc runs but no display says check it is connected key board flashes when first turns on then nothing mouse lifgt stays on any help apreciated forgot to say its running vista home nividia geforce 9600gso not sure of any other specs will try to find out it was custom built