It would help if we could have details of what OS the headless PCs are running?
Whether the remote control is RDP?
Whether the clients are solely the alternate OS to the headless PCs or whether there's some commonality there?
Without those 3 answers above, one can only speculate. However, the leading contenders are:
- Firewall blocking access from specific ports
- Different IP address ranges given out by the two different DHCP servers on the different devices (you really need to unify these ranges and DHCP provision for your sanity)
- ARP issues caused by 2 routers with the same IP address
- Windows 7 vs XP security issues (unlikely if RDP can't work either)
First subsequent question, since you're on the same LAN, is if you can PING one from the other.
Second subsequent question, can you provide an "IPCONFIG /ALL" output from a command prompt from both PCs?