I have an old PC with an 80 GB hard drive. About a week ago, I noticed that when I click on "My Computer/Local Disk" that "Total Capacity" now reads 74.5 GB (whereas before it always read 80GB). I did a disk defrag thinking that would solve the issue, but it did not. With such a small hard drive, I could REALLY use the extra 5GB of space. Any ideas would be appreciated! Ron

The drive will have always come up in Windows as 74.5Gb or thereabouts. It would never have come up as 80Gb.
The only place you'd have seen the figure 80 with regard to that drive would be on the drive's label, the drive's model name and in the computer's BIOS.

The figure 80Gb comes about because the figure is applied as if everything is rounded down, and as the actual figures are 1024 bytes in a Kilobyte, 1024 Kilobytes in a Megabyte and 1024 Megabytes in a Gigabyte, you can see the discrepancy.

Therefore:

For a drive to come up in Windows as exactly 80Gb of storage space, it needs to have 85,899,345,920 bytes of formatted capacity as follows:

80GB is (1024 bytes * 1024 Kbytes * 1024 MBytes * 80) = 85,899,345,920 bytes.

However the drive is advertised as 80Gb being 80,000,000,000 bytes and so the sums work out thus: so if we divide this figure down:
80,000,000,000 bytes / 1024 Mbytes /1024 Kbytes /1024 bytes = 74.50 Gigabytes.

74.5Gb is how it has always appeared in Windows. The same principle works for all size of drives, hence the new Terabyte drives (1000Gb) only show up as 930Gb or thereabouts.