Well.....it takes off some MB's because of the 1024MB equals 1gb but hard drives say 1000MB equals 1GB but I had a 160 and it said like 154.....idk why yours is so low.
shouldnt be, my 200 i just put in picks up 186, so its just the bigger your harddrive, the more you lose, somone told me you lost %10, idk if thats true or not
go into dos (assuming you don't have xp, i'm not sure if it will work in the command prompt) and type fdisk. you should be able to check what your hd is partitioned into...
on another note, my neighbor just bought a 250GB hard drive and windows says it's only 232GB...so i blame windows. (my linux machine shows the accurate HD space)