Ok, some things to do.
1. Do you have Windows Sharing switched on in System preferences/Sharing on the mac?
2. Do you have a shared folder set up on the PC? (Right click on your selected folder, choose sharing, follow the instructions - haven't got a pc to check)
3. Are both computers connecting OK to Airport? And getting an IP address (on the PC, you can tell by the little icon in the system try; on the mac, through the airport menu)
4. If neither are for the moment connected to the internet, try also turning both firewalls off. On the Mac, it's in System prefs/sharing. On the PC with XP and service pack 2, I think it's in control panels.
5. I'll reinterate that the easiest way to test things is to connect the two up via a cable first, see if they see each other, then try wirelessly.
Any of these things any help?