You are not risking anything if you are careful and do not alter anything else than instructed.
I just tested this on my computer. I have civ installed. When I run the setup.exe on my DVD, it asks me if I want to remove Civ. After performing this few steps, running setup.exe asks me to select a language for the installation procedure...
1. Start -> Run -> regedit
2. go to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Uninstall.
Note: Apolyton forum says to go to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall. The correct path probably depends on the options that you selected when first installing civ4 (whether to install for all users or for one user only). Try both.
3. Select {CFBCE791-2D53-4FCE-B3FB-D6E01F4112E8}
4. Right click it, Export, enter a filename, such as civ4.reg. This is for backup.
5. Right click it, Delete, Yes.
6. Try running the setup.exe on your Civ4 CD/DVD. You should be able to re-install now. Yeehee. If so, continue with step 8.
7. If not, you can bring the registry to the original state by double clicking the civ4.reg backup file that you created in step #4. Bummer.
8. Report here how it worked.