Firstly, computers in a Windows network need to be in the same "Workgroup", so you need to give the Mac the same Workgroup name as you use with the PC.
In Finder, go to "Go > Utilities" and open "Directory Access". In the Services tab, double click "SMB/CIFS" and a box opens in which you can enter the Workgroup name.
Now go to "System Preferences" and, in the "Internet & Network" section, select the "Sharing' applet. In there, I have "Personal File Sharing" and "Windows Sharing" enabled.
That should be it as far as the Mac side is concerned. Are you familiar with the Windows side?
When accessing the Mac from Windows, you will be prompted for your Mac Username and Password, but XP Pro should remember these login details, XP Home doesn't.