The network issues are always a big problem
Does the reverse works? I mean if you try on other computers also they deny the access?
It could seem stupid, but are you sure that the workgroup name is correctly typed?
The OS's of computers is XP for all or you have mixed types?
P.s. probably you've already checked
this, NetBios is active and NetBios service is running?
I have found also this suggestion:
"Have local subnet (i.e 192.168.1.0/255.255.255.0) in the Trusted Zone and allow, Direction Both, UDP, Remote: Trusted Zone ports 137 -139, Local: ports 137-139."