I just finished attempting to install MAC OSX on an additional partition (epic fail). After a day of hell I finally got partition 1 (C) set back as active again. The damn XP Recovery cd has no way of setting a partition's activity state so I had to create a FreeDOS boot disc and use its partitioning utility. My partitioning scheme was as follows

Shrink part1 (XP) to 20 GB
Making another 20 GB part out of the unallocated space
Leaving the rest of the drive unallocated in order to make a later FAT32 partition for sharing between the OSes.

Anyway after deleting everything and joining all the unallocated space back into part1 my hard drive shows only 150 GB total space. I am not sure if it shown this before everything, but checking my original dell system configuration via the service tag it shows my system was equipt with a 160 GB HDD, not 150 GB. Where the hell did the

It's probably due to decimal vs binary byte counting. Computers like everything to be in powers of 2. So while we might think of a kilobyte as 1000 bytes, a computer sees a kilobyte as 1024 bytes (2^10), a megabyte as 1024 kilobytes and a gigabyte as 1024 megabytes.

To find the binary capacity of a drive expressed in decimal gigabytes you divide it by 1.024 (the ratio of binary to digital) three times. So a computer will see a 160 gig drive as 160/1.024/1.024/1.024. That gives a little over 149 gig, which is about what the computer will see, taking in account there may be a slight difference due to round-off in the original 160 gig figure as well as a small amount of non-user space devoted to the mbr and partition info.

i totally agree with davincaps. the difference is always about 7%

The math on that is fairly easy but I didn't go into much of it. Let me add the following so I can refer back to this thread if the question comes up again:

1024 bytes = 1 KB
1024 KB - 1 Megabyte
1024 MB = 1 Gigabyte

Therefore 1 GB = 1024 x 1024 x 1024 bytes.
So instead of a billion bytes--1,000,000,000--a gigabyte has 1,073,741,824 bytes
Moving terms around gives:

1 Byte = 1 GB/1024/1024/1024

The 160 gig drive you bought is counting each 'gig' as 1,000,000.000 bytes and not 1,073,741,824. So multiplying each side of the above equation by 160 billion gives

160,000.000.000 bytes = (1 GB) x (160,000,000,000)/1024/1024/1024 or
160,000,000,000 bytes = 160/1.024/1.024/1.024 GB

So when the box says you're buying a 160 gig drive the computer is going to see it as 160/1.024/1.024/1.024 GB (about 149 GB). The 160 billion bytes are still there, it's just that the computer is counting them a little different than we do.