Nothing yo worry about.
The different sizes you are seeing is due to the differences in the way hardrive manufactures and Windows report HD size.
Hardrive manufactures say 1GB = 1000MegaBytes. Whereas Windows says 1GB = 1024 MegaBytes.
So windows would show around 75GB of space with a "80GB" drive. You also loose some space (albeit a very small amount) to file system overhead. (allocation tables, attributes, etc....)