OK, you're stuck with XP after April 2014 because your specialized software only supports XP. And upgrades/replacements are unavailable or cost-prohibitive. What do you do?
* Make sure the system is patched completely.
* Have an up-to-date and working anti-virus on the system.
* Remove any and all extraneous software. Run only the software you absolutely need to run. Don't run other software, especially web browsers or mail clients on the system. Consider the computer now an appliance only for your specialized software. Remove Java, Flash, Acrobat, Air, Office, Shockwave, Silverlight, and any other likely attack vectors.
* Remove any unneeded Windows components (games, Messenger, etc.)
* Disable Internet Explorer (http://pcsupport.about.com/od/browsers/ht/disableiedef.htm)
* It's not always an option, but see if you can run the the software in a non-administrator account. If needed, change the properties on the shortcut to the software so just it, and only it, run in admin mode.
* Have an image backup of the system -- it's likely to break at some point. And since the software involved may not be supported anymore, getting it reinstalled may be difficult. (Even better, install an external hard drive and a copy of Acronis TrueImage or Macrium Reflect and schedule image backups once a week or so.)
* Consider moving the XP system to a virtualized system. That way the system will likely be faster (newer hardware) and can still be used for both general computing (web browsing, emails, Word, etc.) in Windows 7 or 8, and then XP for only your specialized software. And if the XP system is virtualized, backing it up is dirt simple -- just make a copy of the virtual machine files. Windows 7 Pro/Enterprise/Ultimate's XP Mode is good, but note that VMWare Player and Virtual Box have better performance, are easier to administer, and have better access to external hardware. You'll probably be better off using Player or Virtualbox if you can. If the system is virtualized, it's also extremely simple to clone it and roll it out to multiple systems. (Caveat: virtualization generally won't work if your specialized software needs access to serial, parallel, or usb to reach an external device, copy protection dongle, etc.)
* Firewall the system so only the the necessities can pass through. Probably file transfers out, but not in. Or only in from specific IPs (your servers). Lock out common ports (22, 23, 80, 443, etc.) -- you don't need them anymore. If needed, only allow the ports to connect to specific destinations, and not everywhere.
* Allow the antivirus to update, but turn off Windows Update -- it's not going to get anything new anyway.
* Disable any other unnecessary services.
* Look towards any other ways to lock the system down.
So it's all doable. And you can make the system (mostly) secure. But plan on it failing anyway. It's going to be fragile and vulnerable. Reduce your vulnerabilities as much as possible. And don't plan on using the XP system for anything but what is absolutely necessary.