That's two 18GB partitions merged for a total of 36GB. I also have a 117GB program partition and a 127GB data partition.
When given the choice, I tend to install programs to another partition (E=117GB). After 6 years, my 18GB XP partition still has ~6GB free. Moreover, keeping the OS partition size smaller makes it faster to make/restore an image of it and the image takes less space (i.e. fits on one or two DVDs - I also save it to another HDD).
Aw well, I suppose I can use my second other partition (127GB, that would need a really large image size to backup though) if necessary but I don't intend to buy another drive. I already have two (320GB & 500GB) and some of my money will be targeted to buying Windows 7 when it comes out instead.
(Someone is giving me the Vista 64 Ultimate disc. Company excedent.)