How do I take a screen shot?

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Yes it sounds easy and it probably should be but so far it's been a pain in the (blank)! I've been trying to figure out how to take a screen shot for a payment proof for quickybux. Normally, I wouldn't bother (if a site pays it's members, word will get around. Plus I believe this should be optional.) but the site says if I don't I won't be able to receive future payments! I have a Compaq desk top computer and when I googled "How to use the Compaq pc keyboard" (my owners manual was lost sometime ago) I read that I should just press the "prt scr" button on the top right of the keyboard and then save to a folder. Unfortunately pushing this button results in nothing happening LOL! So, I'm totally at a loss here and frustrated because this is a good ptc site and now it's basically useless to me until I am able to post that payment proof. If someone can tell me how this is done, that would be awesome!