most likely, NO. but it's ALWAYS best to backup data before changing something.
personally, i've used Partition Magic to convert numerous (about 6 or so) hard drives from FAT32 -> NTFS with data and operating system perfectly fine - same as it was before converting the HD to NTFS. *knock on wood*
it's still a good idea to backup the data, if at all possible. if you don't want to spend the dough on blank DVDs - or don't have a DVD burner, or don't want to spend the time burning 70 gigs of stuff, etc. then if you have the space, temporarily transfer the files to another HD... in the same computer (or a different one). if i were in your shoes, i'd just get Partition Magic and skip the data backup part. but that's just me. (most all of the data i ever get is easy enough to get back, and wouldn't take more than a few clicks per day to download again.)
good luck.