You can google around, there used to be software you could install that would allow you to uncover some settings from inside windows, if there is any settings higher than the stock settings for the processor. It's not so much that the BIOS is locked, it is that the BIOS is stripped down to the bare minimum, so it pretty much matches the processor, there is very little added anything to even work with, if could get to it.
Even if you do find what you are looking for, and you could overclock the thing, it is still going to be an old slow P4. If you could get it overclocked to...say, 3.6ghz, then you would have a very, very hot running old, slow P4. Time to retire it if it isn't getting the job done anymore.