Does server have virus (see below text)?

Is this a virus on our florida server? One of our PCs (the one of a few with ZoneAlarm installed) keeps getting pinged like below. It increments the port# and retries at intervals ranging from 8 hours to 2 minutes. Or is it some kind of server discovery packet? The incrementing port#s make me think is a virus but not sure. The machine in florida is some kind of NT or 2000 machine.

sample zone alarm message:
The firewall has blocked Internet access to your computer (NetBIOS Session) from 172.28.8.100 (TCP Port 1243) [TCP Flags: S].

Time: 4/16/02 1:27:08 PM