There are clean installs and there are clean installs. What you want to do is COMPLETELY REFORMAT your ENTIRE hd when doing the install, don't keep any exiting file structures. I say this because there is something called the MFT. This can be heavily fragmented. The only way Microsoft recommends you defrag the MFT is by reformatting.
Upgrading is for PUSSIES! You gotta ask yourself AM I A PUSSY??!! Dont be stewpid. Dew a cleen instal! DUH!