Is it WinXP or WinXPPro? Might not be able to do it with WinXP. Check this site out:
Mac PC Filesharing
I have it working on my set up. Just enabled Windows File Sharing on the mac, then added the connection in "My Network Places" on the PC. I have WinXPPro btw....
I was able to mount the pc on the mac by doing cmd-k, and selecting it from the list.
**Important: you must add a "share" on the PC first, or nothing will work. Select your users home folder, right click, then sharing.**