isp says I have a bot sending out spam

I haven't attached a hijackthis log yet because there are two computers on the network and I don't know which one is infected. Do I need to post two logs?

I received this email from my ISP:

"Our Rogers security team has identified that one or more of the computers or network devices connected to your Rogers Hi-Speed Internet service appears to have been infected with a Bot/Virus.

This virus runs behind the scenes on your computer and can be used to send out SPAM messages, attack internet websites, infect other computers, and even access personal files and confidential information on your computer without your knowledge, posting a serious security threat.

Please take immediate action to remove the Bot/Virus from your affected computer(s). If you are unable to remove the virus within the next 48 hours, we regret we may temporarily suspend your internet access to protect your security as well as the security of other customers and our network.

IP 99.240.227.41 seen acting as an Botnet Drone 2013-08-16 00:03:17.
data:
Timestamp = 2013-08-16 00:03:17
IP = 99.240.227.41
ASN = 812
HOSTNAME = CPEbc1401e324d3-CMbc1401e324d0.cpe.net.cable.rogers.com
CNC ASN = 22773
CNC Port = 16464
Vir Guess = ZeroAccess"