Networking problem with XP machines

I have 4 PCs at home and they are as follows:
Larry -- XP Pro SP2; main system (used most often); located in basement;
Moe -- XP Pro SP2; secondary system (has Outlook on it for my email); located in basement;
Curly -- XP Pro SP2; 3rd system (used sporadically during the day); located upstairs;
Shemp -- Win2K build 2195 (unsure which SP it has); laptop used by my wife everyday; located upstairs;
I have Comcast HSI w/external modem, and D-Link DI-604 Broadband Router w/built-in 4-port switch, setup to use Static DHCP.
Larry gets on the internet just fine, as well as Shemp (Win2K laptop). However, Curly and Moe have problems staying on the internet - they will lose connectivity after not being used for a while. On the LAN side, Larry can get to Moe's and Shemp's shared drives, but not Curly's. Moe can get to the shared printer on Larry, but it can't ACCESS Larry's shared drives (but it CAN see them). Curly can't even get onto the workgroup. Shemp can see all systems on workgroup (HOME), and get to printer on Larry, but gets "not accessible" error, along with "not enough server storage to process command". I thought I had all but Curly working before after installing XP on Larry and Moe (I know they were all working with Win2K). Any suggestions would be great.
P.S. I already checked that file sharing is on, I made sure my username is setup on each system with the same password, and that ZoneAlarm allows connections from the IP range given by the router.