Airport Express works from Laptop PC, not from Desktop PC

I have configured my Airport Express and is set up to stream music wirelessly to my home stereo. I am able to configure the ApExp on the Desktop PC but not able to stream Airtunes from it. I am however able to stream Airtunes from my laptop, both via LAN and WAN. I tried disabling the firewall on my desktop with no avail. ApExp firmware and iTunes are up to date. I am running Windows XP service pack 3 on both machines.
Anyone have any ideas??