Are you running Windows XP?
Im not sure if anybody before me mentioned this or if turning on remote desktop connection in the control panel has anything to do with this but you may want to try going to start>run> type in services.msc make sure "Remote Access Auto Connection Manager", "Remote Access Connection Manager", "Remote Desktop Help Session Manager", "Network Connections", "Network DDE", "Network DDE DSDM", and "Network Local Awareness", are all set to enabled or auto (If Auto you may want to change to enabled). Something else to try is to completely disable the firewall or close it long enough to see if that makes a difference, when using a firewall I once had to uninstall it before I could even use the port which I set to block for most and I set to open for some it still would not be allowing stuff to pass through when I set it to allow. You may want to check this on both systems you may also want to disable Internet Connection Firewall in the services.msc.
I don't know if it would help to get SP2 but maybe you could try that if all else fails (if you don't have it already, IF you have windows XP). Also, are you running the same OS on both systems? If not what OS's are you running?
Im not sure if any of this stuff will work since I've never been on or used a network before but it all seems like possible things to try since you are getting nowhere anyway, you can always reset them to default if it doesn't change or help anything. Let me know how it turns out, id be interested to hear.
Good luck!