I don't know about Vista, but at least through XP Pro you cannot use two dots in a file name, and maybe that had something to do with your problem. Through all of my updates of ACR including the latest, I put the old Camera Raw.8bl into a folder called OldACR, in the Adobe CS3 folder. I renamed my old ACR files Camera Raw.xxx, where xxx is the version number. I have never had a problem.
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There is nothing wrong with using dots in file names, as long as the extension is preceded by a dot, such as .exe, .dng, etc. and nothng comes after it.
Using .Bak as a file extension is a way of telling Windows to ignore it, since neither Windows nor Window's programs identify the .BAK extension with any working file extension.
So Photoshop.exe.BAK becomes a file that is ignored in the Windows OSs.
I have no idea why you thought you can't use two dots in a file name. Works for me.
Have you ever seen something like Open office, which has it's version in teh file name of it's exe file, such as OOo_2.3.1_install.exe