XP Won't start

We started the PC this morning - only for it to
constantly re-boot.
If we are lucky we get the blue screen that says it is
checking the disk - it does so, says all is fine - then
reboots.
Sometimes we get a black screen asking how we want to
start windows. If we select safe mode it starts to
scroll files names across the screen then gets stuck and
we have to reboot.
If we select one of the other options it goes to the blue
screen, or to a windos screen that just never loads and
then reboots anyway.

I assume we have a virus (despite Norton Antivirus and a
firewall) What should we do (this is my father-in-laws
PC I am using now)