Acer aspire one in a reboot loop

My acer aspire one Xp edition is in a reboot loop. I think it's a virus. Because my daughters has the same thing the same week. My grandson plays games on them on the internet. And he said he Was on the same sight with both computers. I have tried all over the internet to find a way of formatting them without a disk. And have had no luck. The aspire came with no disk. And my daughters dell laptop windows vista. Has no disk either. Can you help