How can I restore two-way Win 7 / Win XP Connectivity?

[jeh]

My home network has a desktop and an Acer laptop, both running Win 7 Pro, and an older Dell laptop running XP SP3. The two Win 7 machines belong to the same Homegroup and work perfectly happily together. The XP used to fully co-operate with, and be visible from, the desktop but has now partially ceased to do so. It can still ping the desktop, and run Windows Explorer in the allowed folders, but refuses ping requests from both of the win 7 machines, even with its firewall turned off.
I don’t know when this first occurred. All three machines run the same AV, VIPRE Premium. They're connected through the same router, with the XP and the desktop connected via ethernet and the Acer via wi-fi.
I’m baffled. Any ideas anyone? John

 

[jeh]

Quick update. The Ping problem solved - I'd left a VPN connection running on the XP machine. But fixing that hasn't restored the two-way connectivity. Will get back to this later.