Wouldn't you be happier to be nuked and restored and on your way? Wouldn't that be less stressful than worrying about it every single day?
OH MY GOD I FIXED IT.
crosses fingers
I was all set to uninstall and reinstall the K-Lite Codec Pack, which contains libmplayer.dll, but I didn't remember whether it would require me to restart, so I was waffling on whether I wanted to bother this morning. But I Googled Pixie Pack Codec Pack, which also came up on a search for that file and I don't remember ever installing. It turns out that it got installed when I installed Tunebite, which I recall gave me some problems (I feel like it did some freaky things during the install that scared me). That was last year. But I thought, fuck it, let me uninstall Pixie Pack Codec Pack since I don't even need it and never wanted it.
AND NOW EVERYTHING WORKS. All four programs that crashed when opening things—Paint, IrfanView, Outlook Express, and Explorer—now open files with no problems. I have no idea why the issue only presented now and why it was an issue at all, but uninstalling Pixie Pack Codec Pack and getting that DLL file off my system has solved my problem. No nuking required at the moment, thank goodness. And I also feel a sense of accomplishment for sticking at it, researching, and detectiving. Thanks to everyone who listened to me bitch and moan for the last week!