Do yourself a big favor and do a clean install. Look at the people here with problems and the majority have upgraded instead of a clean install, an upgrade leaves a lot of junk behind. The next largest group is with people with outdated PCs.
On a complete install not an upgrade disk it will ask if you to select upgrade or custom. Use custom and follow the prompts and choose format. This wipe out all files. I'm not sure on installing from an upgrade disk. There is a work around you need to follow. It should be posted here some place. I think you install like above but do not enter your key then run disk a second time and select upgrade then.