Not only will the HP Slate run Windows, there's a strong possibility it will run Linux too.
If it runs windoze, it must be 80x86 so it must run Linux, etc.
The gotchas are in the drivers for "strange" hardware e.g. you may find it hard to find a driver for some 3G radio, for the touchpad, etc. But anyone downgrading a flash new laptop from Vista to XP has the same issue.