at my mum and dads theyve got windows (xp i think) the 1 before vista

and theyve got a virus on it.

ive done a system restore from safe mode to before the problem started,
and it helped for a couple of hrs but its now back.

they keep getting a security warning flash up saying the pc is infected do u wanna run a scan, then asks u to buy a program to delete it.

it wont let you open anything up to clean it tho.
theyve got windows antivrus avg (free version)
ad-aware, and norton free versions, but this virus thing wont let them open.
if you try to open it it just comes up with a warning box saying the file is infected do you wanna run antivirus program? and wont let you open it.

it happens when u try and open any program.

any1 got any ideas what i can do?
pls help.

cheers