Go to the Start menu My Computer and it will tell you how large the Hard Drive is. There is always a C Drive and there could be a D drive or other set up as partitions.

It is also possible that there are is a hidden partition installed on the drive. You can check for a hidden partition, most manufactures will place a hidden partition for system restoration and system core operations. The smallest size I can make this
hidden partion is 8MB, it is the smallest that windows will allow it
to be.

Also, another thing to remember is just how much is a real Mega
bite? Remember a 1 Kb is actually 1024 bits and a Meg is actually
1,024,000 bits. I have a 120 GB hard drive that shows 111GB, when you
add up all of the stray bits, it comes out to 120GB mathematically.

You can also check your hard drive size out, just go to the System Information
Tool, click Components, Storage then Drives and take a look at the two
differant numbers reported for Size and Drive Size.

When a hard drive is made, there will always be bad sectors. a 100 gig
hard drive can easily be in the high 90ish gigs.