Why does it seem like I only have 99.8 GB of space, instead of 128?

There are several reasons.

• The file system requires some space for metadata (covered by another SuperUser question). This accounts for most of the "missing" capacity.
• Some of the space may be allocated to a recovery partition.
• When vendors represent the capacity of storage devices, they base their units on 1000 (1 kilobyte is 1000 bytes, 1 megabyte is 1000000 bytes, etc.), but it is common for operating systems and applications to represent size/capacity using units based on 1024 (1 kilobyte is 1024 bytes, 1 megabyte is 1048576 bytes, ...). Units of 1024 are more practical than units of 1000 in computing because 1024 is the exact number of unique values that can be represented in 10 bits. The 1024-based units are also referred to as kibibyte, mebibyte, gibibyte, etc.

As jlehtinen said, there is going to be a 'recovery' partition of 5-10GB, maybe more, they put the recovery image here in case you need to restore your system, so that will be eating up some of the space.

Another thing to consider is that the Operating System, in this case Windows 8, will take up space on your drive, probably around 10GB of space for all of it and updates. If you have Office on there, it will be more.

Finally, when you see that a drive is 128GB, this is stated with the assumption that 1GB = 1000MB, but this is false. 1GB = 1024MB. The numbers given to consumers are Base-10, 1000MB = 1GB, but the computers sees things in Base-2, or binary, 1024MB = 1GB.

To figure out the actual size of the drive, in computer terms, take 128 * .93 = 119GB of storage space. If you consider a 5GB 'recovery' partition, ~10GB for the OS install, then another 1-5GB for additional software. That could easily be ~20GB, and drop you down to about 99GB free space out of the box, and it might be formatted in a filesystem that windows can't read, so you wouldn't notice it.