Can anyone make any sense of this?

• The OS2008's Google Talk "Contacts" tool runs fine on my N800 when I'm on my home WLAN
• The Google Talk aplication (not gadget) runs fine on my Windows PC at work. (So no company firewall blocking of TCP port 443 apparently)
• I can surf the web just fine on my N800 using the "ad hoc" wifi connection I've made on my Windows PC at work. (My wired LAN connection is set "shared")
• However, the "Contacts" on my N800 always report "unable to find online contacts" when I connect via the ad hoc network at work.

So what key config of my work PC's ad hoc WLAN setting have I forgotten here?