Hard Drive Size
Hard Drive manufacturers rate the capacity of their Hard Drives differently than DOS or Windows does. Under DOS or Windows 1 K is really 1024 bytes and 1 M or Meg is 1048576 bytes. But to a Hard Drive manufacturer 1 K is 1K.

so 120gb is (in manufacturers terms) 120,000mb, 120,000,000kb, 120,000,000,000bytes if everything was in 1000s but it's not, it's 1024.

if you divide 120,000,000,000bytes by 1024, you get 117,187,500Kbytes (real),
if you divide that by 1024 you get 114,440Mb (real),
then if you divide that by 1024 you get 111Gb real gigabytes. (which is what windows is telling you, as the real GBs)

which is why you're loosing some of your "120gb", and only getting 111gb. because the manufacturers of Hard Drives like to inflate their hard drive sizes.

basically -
1 bit = a single digit, either 1 or 0
8 bits = 1 byte, a combination of 1's and 0's
1024 Bytes = 1 KB (kilobyte)
1024 Kilobytes = 1 MB (megabyte)
1024 Megabytes = 1 GB (gigabyte)