A 320GB hard drive will only appear as a 300GB. That’s due to the decimal vs binary difference. Computers work in binary but hard drive manufacturers prefer to measure their hard drives in decimal because they sound bigger. So, for example a gigabyte in binary is 2^30 or 1,073,741,824 bytes. In decimal it’s only 1,000,000,000. 320GB in decimal is really only 298GB in binary. HP also reserves several gigs as a hidden partition, which many manufacturers do now, as they claim it’s pricey to ship Restore Discs with each new computer, though the user can create them as well.
PC Guide explains this more thoroughly.