it is a good question BTW. First the manufacturers of hard drives since 2002 respecified Kbyte, Mbyte and Gbyte and they came up with “very smart” solution to call Kb =10e3, Mb=10e6 and Gb=10e9 respectively. It allows them to sell drives as higher capacity, as the real capacity is to be measured in a real Kilo, mega and Giga as K=2e10, M=2e20 and G=2e30 respectively.
The difference is quite significant as for 100Gb drive the presupposed capacity would be
100x2e30= 107374182400 when the “their” capacity is 100x10e9, that is some 7Gb less !
After that, not all the drive capacity is accessible to data as drives aren't used as RAW drives normally, but require to be formatted. Formatting drive means introducing the system of coordinated to the drive geometry so the a position of the some particular byte may be specified to the driver which will address the data on a drive. As you can imagine, the whole space has to be mapped so it takes additional average 7% of a drive space. So it is where your gigabytes have vanished.

Addressing second question, I'm not sure what your definition of “lot a trouble is”. For my taste it had never worked before in a fashion it wouldn't drive me out of my mind and for the reason I abandoned PC platform completely and use OS X on Apple hardware exclusively.
But is entirely different subject. You can try Mandrake Linux, so far it was most polished and relatively peacefully coexisted with Windows (and easy to install in another partition as Mandrake can do it automatically).

Hope it helps.