Good question charles_scott. I just know that when people advertise drives, when they say 80 GB, they really mean 80 billion bytes, which is a bit different. Since you go bytes -> KB -> MB -> GB, 80 billion/1024/1024/1024 is how much you should have in GB (it equals something close to 74.5 GB).
As for your 78 GB total... for some reason they gave you 83.75 billion bytes perhaps!?
AMD Athlon 64 3000+ socket 939 @ 2.3 GHz
MSI K8N Neo2 Platinum motherboard
EVGA GeForce 6800 GT
2 GB DDR 400 (PC 3200) (2 x 1 GB sticks) (Dual Channel)
DVD player and seperate DVD RW drive
80 GB Western Digital Hard Drive
500 watt power supply
Sound Blaster Audigy
Logitech 5.1 surround sound speakers
SAMSUNG SyncMaster 710T LCD monitor
Aspire X Navigator Case