Here is what you can do. It's a PitA, but it's worked for me, and I've re/installed WoW no less than 20 times on my computer, my wife's computer, several friends' computers, and even my mom's comp!
Uninstall WoW completely. Make sure you delete the 'World of Warcraft' directory as well. Depending on which version you have, it will sometimes leave the /interface and /wtf directories.
Once you start from scratch, the trick is to install WoW, and then reboot between every patch. That will clear your temp directory and the system cache, so, if Blizzard named any temp folders in a later patch the same as they did in an earlier patch, you won't be picking up erroneous data.
Obviously, you'll save yourself a LOT of time if you get a 3.0 client install disk. I don't know if that's possible, however. Me: I just install my vanilla client, then tBC client, then WotLK client, and only then rely on the downloader for patches.