Just a guess that you can check out...
I've seen cases when the DHCP client on an XP box will tear down the entire TCP/IP stack at 50% of the lease time exactly to perform its lease renewal. Moreover, this client will always attempt to get a new lease everytime it starts, even if it still has "time" on the lease it previously held.
When that happens, all active connections are dropped, including shares, and the mapped drive connections will appear as "disconnected network drive" in My Computer until they are clicked on again.
It would be interesting to check the disconnects against the DHCP lease time to see if there's a correlation.
There are also timeouts for NetBIOS connections -- What settings have you tweaked? -- I've always has more success using a protocol like IPX/SPX over TCP/IP on a LAN because TCP connections are incredibly sensitive to timeouts. Moreover, the TCP/IP settings that would work well over the internet and your LAN may not be the best settings to use for shares, so you may have to strike a compromise to get the results you want. Keep in mind that if you are using NetBIOS over TCP/IP that the TCP settings may override or conflict with the NetBIOS-specific settings.
Lastly -- Networking: Firewalls on the PCs may be blocking essential network level broadcasts and other communications that the PCs need to make to stay connected to each other. On that same token, some switches/hubs do not properly forward/flood MAC level broadcasts in an attempt to prevent broadcast storms. So check your "security" options.