This can sometimes happen when you are installing the retail version over the top of the old beta versions. You should completely deinstall - or better --format the hard drive - then try to do the installation again.
For my home systems I will usually take the opportunity to buy a larger/faster hard drive. Then I will remove the old hard drive, put the new one in and build the system. Then I will reattach the old hard drive as a slave. Then I can copy all of my files over. When I am done I completely reformat the old drive and re-organize the space.
This was first published in February 2001