Here are my thoughts:
1. Even though my "main" HA comptuer is running Windows 7, XP is a lot easier in many ways.
2. Most modern PCs have a BIOS option to power up when power is introduced (like when power returns after a power outage, or when you just plug the box in, even after a graceful shutdown). That should take care of the system booting up after an outage.
3. A small UPS would help carry the system across "short" outages, without it having to reboot, etc. This would also allow for the system to be shut down gracefully during a longer outage.
4. There are tweaks for XP to have it log in automatically at boot up. Instructions should be pretty easy to find using your favorite search engine.
5. If you want to keep timers and/or macros stored in the CM15A, then a Windows build with AHP is really your only option. If that isn't an issue for you, there are third-part programs out there, for Windows, Linux, and Mac, that can do a who host of different things, and control the CM15A or CM11A (in some cases ONLY the CM11A) directly.
Whichever way you decide to go, understand that nothing is going to be foolproof. The CM15A can lock up, the PC can lock up (or both can happen at the same time), taking your system out of commission until it is fixed.
There are utilities out there for XP (and I'm pretty sure it is built into Windows 7) that can detect if the PC switches to UPS battery power, or back to AC power. Those events can be used to trigger other actions (after a power failure, turn on certain lights, send me an alert e-mail, etc).
I am using some of these to send me e-mail alerts in the event of a power failure (my router is on the UPS, too). I played with using the same method to send some AHP commands using the SDK, but I never implemented that part. All of this was done with some simple batch files.