At the risk of prologing the thread, I consider the term "PC" to usually include Windows as part of the package. I suspect lots of other people do too. Anyway, as long a debate as the "what is a PC anyway?" question can cause, it wasn't what I want to get at. I percieve Windows itself to be the basic problem. It is because of Windows - and software written for Windows! - that PC's shouldn't be used for important HA functions.
I have yet to come across any decent windows-based HA software, at least any that doesn't exceed my pain threshold. I don't know why, but I'm guessing that the developers get distracted in trying to make it look cute or get the layouts right etc. The basic fundamentals always seem to be neglected somehow.
I've tried a bunch (and bought many), including HCA, HS, Powerhome, CQC, Houselinc Desktop, AHP, etc, etc. They all suck in one way or another, many in several ways at once. Or they miss critical functionality. Or they're a real pain in the butt to use once you get beyond trivial stuff. Or unstable. etc.
I use several "PC"'s for HA control, but not with windows. The hardware is chosen very carefully. Software even more so. I have grown to like no-moving-parts hardware, such as the mini-itx boards with no fans and booting from compact flash. Use a DC powered model with a 12V battery in the enclosure with it and a decent OS and you've got something that will outlast most of the rest of the gear in the house. With serial port controller in the PCI slot you've got more than enough options to add extra hardware. Don't expose it to the 'net and it'll run without hiccups for years.
My Insteon/UPB/X10/rain8net/w800rf32/etc controller is one of these. No display. No GUI. Custom software that does exactly what I need/want - nothing more, nothing less.
Even on "PC" hardware.
PS: a special thanks the the HCA folks for first starting me down this path!