I would imagine the HAL is determined at install time like Win2003. Meaning if it detects 2 processors it installs a MP version of the HAL. If it doesnt, well then it gets a single version.
There is a way to configure for the MP HAL even if the SP is installed after the install. But the procedure looked cumbersome on Microsofts website at the time I was looking at it.
I can verify in Windows 2003 if you add more cores with the MP HAL it will pick them up.
Windows 2008 doesnt have this limitation and I would guess Vista as well. You can switch between MP and SP and the OS will install the processors as you add them.