Speaking about accessing, I had similar problems..
You have to first make sure that your anti-virus doesn't block your firewall.. (Which I notice it does.. ) And sometimes the win 7 firewall too... Then just the basic, you all need to be in the same workgroup...
Try setting a IP adress eg: 10.0.0.x or 192.168.0.x then from cmd try to ping each other.. if the ping is there then something is blocking it.. IE: firewall...
I also read somewhere that they said:
network sharing centre -> change advanced sharing settings -> you have to change the one bit from 128-bit to 54-bit.. As they (XP) can only use 54-bit...
from the XP computers, i don't know if this is going to help, but after everytime you change something, you need to go to network place, on the left hand side there is a thing call "setup home or small office network.." just set it up.. and then when they ask you for sharing, say yes.. and workgroup type your workgroup in.. (MSHOME or workgroup..)
and then just finish..
Hope that helps.. I know how it feels NOT being able to LAN..
if you still have the problem, ask and I'll try other options.. as I also had this problem before when I LANNED ^ ^