Which version of McAfee you are running? is it 2009 or 2010?
Also get the IP address of both PC's as well as Router. We need to add it on Firewall's "Networks" portion to get this issue resolved.
I purchased 3 user Total Protection from McAfee Online on 2/21/2010, so I assume it is 2010.
Router is a Linksys W#RT54G: 192.168.1.1
Server: XP Pro, 192.168.1.4
Celeron 2 (workstation): 192.168.1.7