Vista Home Premium 64 bit
Have you thought about upgrading to Windows 7? It's pretty much the same as Vista only much better.
I have a suspicion of what might be the problem but I could be wrong. User Account Control in Windows. When I first installed CoH I was getting an error when every time I updated the game to the new patch. I found out the UAC was preventing it from writing files to the proper folder.
There are 2 things I can think of that you can try:
1) Click your the Windows icon to open the start menu. Type in the search box 'UAC' and click it in the list. Turn UAC completely off.
2) If you don't want to turn the UAC off then choose a different directory to install Mids' to. The problem is when installing to the default location the UAC is preventing certain files from changing/being added. Just create a folder somewhere other than Program Files and when installing Mids' choose that folder to install to.
One thing I have noticed is when you uninstall Mids's it does NOT remove the folder with your saved builds. The folder with the saved builds is by default in the Windows Documents folder. For Windows 7, and I imagine Vista as well, it's located in: "C:/Users/(user name)/My Documents/Hero&Villain Builds" (it shows up in Mids's options as "C:/Users/(user name)/Documents/Hero&Villain Builds")
You can change the default save location for your builds also. Click 'Options' in Mids' then click the 'Updates & Paths' tab. Click 'Browse' where it says 'Default Save Location' and select a new location. (You could move the default folder with the saved builds and just point Mids' to its new location.)