overclockers pre built question - operating system

[ThriceNightly]

http://www.overclockers.co.uk/showproduct.php?prodid=FS-180-OK&tool=3

with the pre-built systems if i choose the operating system at the same time is it pre-installed or do i have to install it?

i.e. is the installation of the operating system included in the price?

 

[ThriceNightly]

thanks