Can I (legally) repair a Windows XP install without a copy of Windows XP? Restoring from the OEM recovery disk and wiping the applications is not particularly appealing, especially since I'd like to recover application settings. Am I expected to buy a copy of Windows XP in order to do this? Are they even still selling them?
If your Windows install key/license is legit you can use any compatible media to reinstall however you'd like.
The vendor should have provided a Windows XP CD-ROM (preferred - vendors who do this are providing a better service in my opinion), or some sort of a process to create a Windows XP installation/recovery CD (I regard vendors who do this as "cheap").
There are a few products which will likely be of interest to you:
Drive Snapshot (full disk imaging)
Magic Jelly Bean KeyFinder (to recover your Product Key in case your sticker gets lost)
Before you do anything else with your machine, use Drive Snapshot to create an image of your hard disk. That way, if your experiments go wrong, you can get back to where you were. The beauty of a backup image that you can "snap back to" is that you can do risky stuff with your experiments to really get the answers you need since you don't have to worry about "losing the [whole] farm."