when running PT LE on "nehalem" or i7 processors there is no option to select using less then the maximum number of cores in the setup menu.
this causes errors and stability issues and leave not enough breathing room for the windows os for backround stuff.
on PC you can disable hyperthreading in the "bios" and then "unlock" the cpu options in PT.
since there is no "bios" on macs, the only way to disable hyper threading running windows on a new mac pro "nehalem" using bootcamp is as follows :
right click "my computer" > properties > advanced > startup and recovery > settings > edit
you have to edit you boot.ini file:
first, copy the last line and paste it under so you'll have two identical lines.
this loads up a dialog on start up where you can chosse what line to use so in case the new one fails you can recover.
now, on the new line you created add:
"/numproc=8"
save
mine is an 8 core machine so my only option was to work with 16 cores.
after making the change to the boot.ini file i can select between 1 thru 8 cores to be assigned to PT.
if you have a 4 core machine just add: "/numproc=4" as the number of your phisical cores.
http://support.microsoft.com/kb/170756/ (scrolll down).