I just recently started using ccleaner and think it's great. But tonight I noticed that all the .dat files on my XP pc now showed a Windows Media Player icon. The files weren't actually associated with WMP, i.e., double-clicking on one would produce the "what would you like to use to open this" dialog; and the File Associations section of Explorer had no entry for .dat.
I did a bit of Googling and learned that this is a common problem, and one many people have a lot of trouble solving. I tried a few things which didn't help, and had a somewhat more complex solution to try next, when it struck me that I had run ccleaner for the first time a few nights ago.
I restored the registry using the ccleaner backup I had created, and sure enough the .dat icons reverted from the WMP logo back to the typical "generic" Windows icon. (Ironically, several of the posts I found about the .dat-to-WMP issue had said things along the lines of, "any good registry cleaner should identify the source of the problem and fix it.")
Out of curiosity I ran ccleaner again, and found that the problem was in the "Unused File Extensions" option in ccleaner. Sure enough, ccleaner lists .dat among the many unused extensions, and the "fix" message says, "The file extension .dat references an invalid program identifier. These are often left behind after uninstalling software. Solution: Delete the registry value."
Well, obviously in this case that's not a "solution"! .dat files may not identify a program, but they are certainly "used"! Perhaps an advanced user should have known this right away, but still I think it would be useful for ccleaner to add something along the lines of, "unless you are sure this extension is not used for any purpose, do not proceed with this fix". Other extensions listed by ccleaner as deletable included .tmp; .bak; .regkey; .ax (used for video codecs, I believe), and dozens of others.
It's still a mystery to me why deleting the .dat extenstion from the registy made the .dat icon turn into a WMP icon. But I'm wagering that lots of folks who have encountered this "annoyance" did so after using a registry cleaner (not necessarily ccleaner). I can't possibly be the first person to encounter this from ccleaner, especially since I see that the same thing happened on my other pc, where I installed and ran ccleaner at about the same time. However, I couldn't find any posts about this during a brief (likely TOO brief!) search through this forum.
As much as I like and appreciate ccleaner, I won't be using the Unused File Extensions portion of the registry cleaner any more!