Per Microsoft:
1K=1024
1M=1024K
1G=1024M
Therefore, 1G=1024*1024*1024=1,073,741,824
Per hard disk manufacturers:
1G=1,000,000,000
Therefore, a hard disk manufacturer will say a Microsoft 100G disk as 107G.
This is the main cause of the difference between Windows and what's printed on a hard disk.