Answer

This matter would be best explained with the following: (Take 160GB HDD for example)
1. Basically, a 160GB HDD (with regards to any manufacturers) has 149GB of effective storage - which means that only 149GB can be utilized. For computation: If 1GB=1024MB 1MB = 1024KB 1KB=1024Bytes
Therefore 160GB 160,000,000,000Byte/1024=156,250,000KB 156,250,000KB/1024=152,587.890625MB 152,587.890625MB/1024=149.01GB
Note: Same principle applies when using different hdd capacities with respect its manufacturers: 80GB,100GB,120GB,160GB,250GB,300GB,320GB,500GB hdd.

2. In order to verify the actual capacity storage of the hdd, you can go to START --> CONTROL PANEL --> ADMINISTRATIVE TOOLS --> COMPUTER MANAGEMENT --> DISK MANAGEMENT --- Then you will be prompted with the screen (please see pics).
a. If it so happens that your HDD has hidden partition (normally recovery OS are placed here - so please don't delete this for future recovery use), there will be added partition slot in the disk management which you don't normally see in My Computer or device manager.
b. If your computer don't have any hidden partition, then you can directly calculate the total capacity storage of your hdd by adding them up.

3. Lastly, when a 160GB hdd that has 149GB effective storage is being breakdown into 3 parts, its calculation will be like this: 160GB total effective storage (149GB) = Hidden Partition (3.90GB) + C: (72.33GB) + D: (72.82GB)

Note: the same calculation principle applies to different hdd capacity.