What OS is your Mac (10.x.x)?
Do you have the Mac firewall turned on? If it is on you could be blocking your own access.
Can the Mac ping the XP machine? To try a ping got to the Mac’s Applications folder, then Utilities, then Network Utilities application. Choose the ping tab. enter the IP address of the XP box and click start. To get the IP of the XP box: Goto the Start menu choose run, type cmd and hit enter, in the Command window that opened type ipconfig /all. This will present you with lots of information the main points of interest are the IP address and Default Gateway.
If you can not ping the XP machine from the Mac, verify your Mac’s IP settings by opening your System Preferences and verifing your Networking settings for active network adapter. The active adapter will have a green dot next to it’s name. You IP address should be the same except the last 3 numbers, and the gateway should be identical. if they are not then something is wrong with your network settings on the Mac.
At this point we need to see if it is connect wirelessly or via an ethernet cable or both. If wireless are you sure you are connect to the right network. If wired are you connect to the same network switch/router as the XP box?