OldHotelman had it right - at least in my case. HPLJ4250 installed locally on a w2k machine, XP and w2k PCL6 drivers only. I tried to install on NT4SP6a, IE6sp1, pointed the install to the local PCL6 inf, and got the printer driver unknown error.
Extracted the NT4 PCL6 locally on the NT4 box...