Depends how good a 6yr old pc it is but they guess you need a Pentium 2 with 400MHz + processor to run games for 286 using DosBox so the chances are your computer will not work well in DosBox.
However, I take all my screenshots in Irfanview (can't be bothered to find the link, Wael posted it above) and I only very occassionally have trouble with it. That would be my recommendation for taking screenshots of anything running in Windows.
Obviously, if you are running games in proper Dos it won't work because it's a Windows program. It should work with anything running in a windows dos shell though!
Hope that helps.
