XBMC is far less OS dependant than any other HTPC-software I've ever worked with (and I've tried 'em all). That XBMC's strongpoint.
Take Mediaportal for instance. It heavily relies on the dotnet Framework. The configuration module takes "ages" (relatively speaking) to start and so does the main program. If a developer writes an 100 kb plugin that requires framework 3.5 you'll have to install the huge framework 3.0 plus package. Granted, modern-day HD's are large, but still the HD needs to seek through hundreds of framework files to get it's plugin working and that takes time (seek times are the Achilles heel of an HD).
I've been using an dedicated HTPC for ages. It's an old Celeron 1300, socket 370 (that's pre WWII stuff
). When using a standard XP installation, it takes some 40 seconds to boot. Using a slimmed down version of XP it needs 15 seconds, only using some 80 mb of memory. When resuming from standby (S3) it only needs a second to be fully operational.
Desperz uses no SP version. That's the beauty of it all. If all of your hardware is working, why bother to install an Service Pack. XBMC doesn't need it, only other programs using the OS do.
- Why bother to use the DHCP services if you can use a Static-IP.
- Why bother using the Theme services if the OS remains "hidden".
- Why bother using Universal Plug en Play if all you hardware is already installed
- Why bother to use the login screen or fast-user switching if there's only one user?
These are just a few things to make an OS start faster by using less resources, dependencies and disk-seek times.
If you're trying to boost your OS, I'd go for XP. It's been there for years and when slimming down your XP, you can use nLite which is far more developed than vLite for Vista. At this moment vLite doesn't even have half of the cleaning/tuning options that nLite has.