Can the print server ping your desktop?
Can the print server access your external hardrive?
Ping is unrelated to storage access. This is difficult to diagnose fully without knowing every detail about the network, but right now I'm seeing two possibilities:
1) Some kind of wonky routing issue. Assuming the router is handling your WAN connection, and given that the Macbook is able to access the printer, the most likely scenario in this vein is that the problem is actually a misconfiguration between your desktop and the router. The expected outcome of this is that the XP machine would be unable to access the network storage as well.
2) The more likely scenario is that your pings are being dropped for an unrelated reason, probably by your router due to filter rules being set too broadly in the firewall. If that's the case, than the problem is actually getting OSX's implementation of Samba configured properly. That you can see the printer suggests that it's working to some extent (printers and storage devices are both typically shared via SMB), but there's a problem regarding accessing the storage itself. It's probably permissions related, but as I have very limited Mac experience I can't offer specific advice on how to resolve that. Perhaps one of our Mac gurus will be able to guide you further in that regard.
Incidentally, if you want to free up your laptop, you could look into something like
this to connect your printer to your network.