I'm not on my XP box right now, so I'll do the best I can from memory for the XP part:
On the Mac...
1.) Open System Preferences. Open the Sharing pane.
2.) In the Sharing pane, go to the Services tab. Check Windows Sharing. Note the address following "Windows users can access your computer at...".
On the PC...
1.) Go to Start Menu. Click Find.
2.) On the left, choose "Find People or Computers".
3.) Search for the address you noted above. (Should be something like "\\your.ip.address.here\yourusername")
4.) The Mac *should* show up in the results, Double-click it, enter your password.