That's two of us retired. I did it almost five years ago.
Now back on topic. My way since Vista's release screwed up my install then,
1. Uninstall the game through Control Panel, yeah I know but a clean slate works best.
2. Delete all the Medieval2 folders you can find including your copies, and assuming you installed in default folder with UAC enabled,
a clue to where the hidden one lurks.
3. Run a registry cleaner like Ccleaner. This gets the garbage CA's uninstaller does not remove.
4. Restart your computer.
5. Be sure the UAC is disabled.
6. Be sure you have taken ownership of C: drive.
7. Install on C:\Games. If you have a second drive, better yet.
8. Test vanilla and Kingdoms. Are we there yet?
If the answer to 8 is, "Yes", to be continued. If the answer is, "No" what error messages are you getting preferably screenshots?