As 'Hdthree' it could be a compatible issue with Win7, corruption or (what I think) your encountering access/privilege problems.
Windows 7 Beta is known not to be uncompatible with World of Warcraft, you need to update Win7 if you are using this.
If not...
1. Right-click and 'Run As Administrator' on the installer.
or
Default:
World of Warcraft installs to: "C:\Program Files\World of Warcraft\"
The Burnign Crusade installs to: "C:\Program Files\World of Warcraft\"
Wrath of the Lich King moves the WoW folder and installs to: "C:\Users\Public\Games\World of Warcraft"
It apparently advises you to do this to prevent problems with patching. If using the Expansions or updating note the "C:\Users\Public\Games\World of Warcraft" can be restricted by Windows 7.
Windows 7 can also prevent some renaming of files done near the end of the install progress.
The good news is WoW doesn't setup the registry in the installer until you run 'wow.exe' to play the game. This means it can be easily manually installed.
1. Start Windows in Safe Mode (to prevent Microsoft issues).
2. Uninstall World of Warcraft. After completing the uninstallation, open "My Programs" from the "Start Menu" and delete the WoW folder left behind by the uninstallation.
3. Create a new folder on your Desktop. You might call it 'wow installer', or something similar. You can do this by right-clicking the desktop, clicking New, and "Folder".
4. Put "Disc 1" of World of Warcraft into your computer. Do not start the installer. Instead, Open "My Computer", right click the CD drive, and click "Explore".
5. Copy and paste the contents of the World of Warcraft CD to the new folder.
6. Copy and paste the 'tome' file from Disc 2 to your computer. Do not copy the entire CD. Instead, select 'tome', copy it, and paste it to your new folder.
7. Repeat the previous step for your remaining CDs. Remove Disc 2 and copy each 'tome' file from each of your other CDs (i.e. Discs 3-5).
8. Open your new folder and run the installer program. The game should install as normal.
(This procedure also works for any expansion pack)
Tech Note: World of Warcraft does NOT install to the Windows registry. You can actually copy the entire installed World of Warcraft folder from any location on your hard drive to another location, across a network, or other transfer medium. You can copy it to any operating system, including Windows, Mac, or any flavor of Unix/Linux without reinstalling it again. Great way to avoid having to re patch after install.