I use <a href="http://www.faststone.org/" target="_blank">Fast Stone Capture</a> (is cheap, life-time license and includes a video recorder) Of course, I use NS2 windowed.
Enable full screen native resolution (to be safe not to ruin your settings) then change it to windowed in the same native resolution, this should give you a borderless NS2 which takes screenshots. Next time you enter the game it will be bordered, just set it full screen and windowed again and your native full screen resolution will be borderless windowed on the full screen, it will let you capture full screen shots that way.
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Easy work-around is to download <a href="http://www.fraps.com/setup.exe" target="_blank">FRAPS</a> and use it for screenies :)
Would be good if NS2 had a screenshot button which saved them into a folder.
but the game itself its all graphically bugged here though
Of course, I use NS2 windowed.
Example:
<img src="http://a.imageshack.us/img843/8624/20100801162839.png" border="0" class="linked-image" />
<img src="http://a.imageshack.us/img44/3962/20100801162155.png" border="0" class="linked-image" />
(version 2.9.9 build 8086) let the fraps window open don't minimized it ;)
i also have dual monitor perhaps this is the reason :/ (i don't test with only one monitor ^^ )
good luck :)