I'm sorry that I just noticed this topic. I would have offered help sooner.
My first thought when I read the first post of this topic was firewall. You'll need to set the IP addresses for both computers as safe or accepted or whatever in ALL of your firewalss on BOTH computers. If you have winXP with Service Pack 2, you'll need to click your advanced firewall settings and add your IP address to the allowed IP address section. If you do not have winXP with SP2, then just turn the windows firewall off.
In your other firewalls and internet security programs, please set your IP address to the allowed section as described in your softwares help files.
Again, as everyone has suggested, be sure to set the disks, directories, or files you wish to share as described in the help files reguarding network file sharing.
I'm sorry not to be more specific about the required steps but with all of the software possibilities and OS choices, it has gotten difficult to know how each system works. I know what to do but without sitting in front of your PC, I can't describe the process blind.
Hope this helps you out.