you can't just use it as a bootdisk; if he has Win9x or ME there are other solutions, like avast's DOS-Scanner (imho free).
for NT/2000/XP avasts's BART-Cd would be ideal.. (don't know if the free trial is still on, though...)
there are lots of other reasons for modem problems than just viruses; why not not try a normal install of avast ? you might do it in SafeMode (F8-Boot) & include the avast Cleaner, too (products page)