Maybe, just maybe, you'd be better off using the environment variables on XP and running your clipper program natively.
I've come across very weird problems with the clipper/xp combination, but the solution always came down to editing environment variables, using the program in full screen mode, messing with cmd line shortcut properties, using Win9x compatibility mode or mapping network printers correctly.
Of course, if you are using the 64bit XP version them simply forget it.
If you are going for the emulation rote, you could try VMWare/VirtualPC too and see if you have any luck with them.