Right Click My Computer>>Properties>>Computer Name>>clear "WORKGROUP" and enter in your home workgroup name. Exactly the same as you would in XP Pro. Unless you have other things configured on the Server 2003 OS (like AD and Certificate of Authorities), that is how it's done ... simple.
Now if you want users from their XP pc's to access the server for things like file sharing, then you would create user accounts on both Server 2003 and XP that are the exactly the same. Then trust the ip address you assigned the server in IE on your XP pc's.
One last thing ... you'll need to "share" the drives or files on the drives on the server so your XP pc's can access them.