This might do it:
"Got it to work.... whew!!
1) had to pull and erase/reformat the disk to get rid of Dell Diag partition and other junk
2) re-booted and hit F12
3) selected Diagnostics (which now appears... before it did not)
4) run Diags.... when it completed, it asks if you want to boot from CD
5) inserted Windows boot disk into DVD and - Presto - it booted
What a lousy way to have to make this simple think happen, but, hey -- it a Dell!"
I couldn't have said it better.