mmm... try using DosBOX. Ctrl-F5 gives you a capture in .PNG-format. From XP, usaually it's alt + print screen, but this might only work in windows native programs. You could use n external progam (IrfanView for example) to take the shots!
More about screenshots in
the screenshots thread! in which I just took 15 screens or so from the nice little game pushover! (using DOSBox)!
edit: I did this before reading your post, btw.