According to hard disc manufacturers, 1 kilobyte = 10^3 byte = 1000 byte. According to most OS vendors and RAM makers, 1 kilobyte = 2^10 byte = 1024 byte.
This means that what a hard disc vendor calls one terabyte is 10^12 bytes (12=3*4) while according to the other way or counting one terabyte is 2^40 (40=10*4). So, one terabyte according to method one is about 931 gigabyte according to the latter method - assuming I haven't messed up my calculations You also lose a bit of space for overhead (or I guess you could call it accounting) for the file system to keep track of where the files are, what disk space is available, file names, creation dates etc - either this is small enough to dissappear in rounding or (more likely) the disk is slightly bigger than one terabyte.
Unfortunately, I can't help you with the original problem - I'm not very familiar with Windows.