How do you take a screen shot when Alt-Print Screen stops working?

This is an error that happens to me at least twice a week for years, and no one seems to have a solution. I’ve read some of the help pages on this in web searches, and none of them help.
   As far as I can tell, only one case of this has been reported, but, sadly, the original thread cannot be readily found any more. Most have turned up in fake support sites. (The search phrase is Alt-Printscreen And Ctrl-V Only Works 3X—while I have not counted mine, I’d say this isn’t far off the mark.)
   How do you do a screen shot in Windows (XP or Vista) when Alt-Print Screen stops working?
   And, how and why does it stop working?
   Usually the solution is to reboot. Sometimes, however, rebooting isn’t a desirable option—such as in cases when I need a screen shot of something that I know I won’t be able to get back.
   Let me state some of the things that have been posted by well meaning people elsewhere that have not worked for me.
   Clearing the clipboard doesn’t work. (The usual copy and paste commands still work with text though.)
   On this computer, I don’t have a Function key, but even on my laptop, where I do have one, it’s not activated.
   I don’t have a second monitor plugged in.
   I haven’t installed Boot Camp, whatever that is.
   I don’t think it’s a memory issue because I have oodles more memory today than I did on my old machine.
   I don’t use Microsoft Office—I say this as some of the advice is around how an Office installation will screw up Alt-Print Screen.
   I used to think it was Photoshop, because, on my old computer, I could shut down the program and reopen it, and Alt-Print Screen would usually work again. Or, I could shut down one version and open another. However, these tricks have now ceased to work, though I have a feeling that they once worked on this machine.
   I’d like to say it is Chrome-related, because I have never been able to take a screen shot of the ‘Aw, snap!’ error page that comes up frequently. Whether there’s something related to the graphics, the crashes and Flash, I don’t know. Browsers crash so many times a day now that it’s hard to pinpoint whether the crashes are the cause of Alt-Print Screen failing, or whether the command had failed beforehand.
   But I’ve had these problems long before Chrome was even released, so we can’t blame Google (again).
   I’m putting this post up in case someone has some suggestions and future computer users can find this.
   The closest I have found to a real explanation is from Adobe, on a related issue:

That sounds a bit like an old Windows bug where the OS would stop informing Photoshop of changes to the clipboard after a while (we never determined the cause, but saw the same thing happen in other apps, and reported the problem to Microsoft).

   And, please, no unhelpful ‘Buy a Mac’ comments. You guys are seldom around when I Tweet about Mac problems (believe me, they screw up just as often, with everything from missing icons to stuck DVDs to files that disappear mid-transfer to fonts that don’t show up in a PDF even when subsetting is on …), and the Windows people never say, ‘Buy a PC.’