Thanks!
A little note: XP Mode needs Hardware Virtualization to be enabled in the BIOS. If it isn't, XP Mode won't start and will display a message instructing you step-by-step on how to enable it and then to cold boot the PC.
Well, do it... it took me several warm boots, and several failed attempts at diagnosing what was wrong to make up my mind and read the whole thing. Serves me right for complaining that users never read my message boxes, I guess.