the reason people do this is because if windows fusks up and you need to format and reinstall you can just format the partition and not loose data, you will still have to reinstall all your programs like office and such,
IMO its easier to just get another HDD to put your data on, partitioning just adds 1 more thing the can go wrong, if the partition table fails you are pretty much fvcked, cause that will hose windows and your data, you can get it back but its a pain in the ass
I used to do it till i had a partition table die on me

damn that sucked
now i just give windows its own 40 gig HDD and I install all my programs and such to it
all other data is on one of teh 3 other HDDs in my sys, and the important stuff is mirrored on all 3