IT koutek
The simplest way is to call Dell assist center, but they require to verify, you are an original and registered user. But there my be a several legal reasons, why it my be a problem. For these reasons I come up with this little handbook.
To ged rid of the unwonted and unknown password you need to reset CMOS to defaults, to do it, you have to short-circuit a specific chip on the motherboard.
You are going to need small screwdriver, Dell D600 notebook and paperclip or small scissors.
To reset the CMOS memory, where is the password stored, you have to short-circuit the chip containing CMOS. I would say the hardes part is to take apart the notebook. In Latitude D600 model the chip is located under the PCMCIA slot under the black plastic as can be seen on the first picture bellow. I will not describe here the procedure how to take the notebook into parts, I believe you can help yourselves.
Once you get to the proper chip you need to short-circuit it's 3rd and 6th pins, as can be seen on the second picture bellow, while powering on the computer. It my seems, you need as less three hands, but don't worry I made it with my own two:).
After powering on there will be a non standard message on the screen, something about pushing Fn-X, that is a goog statement and everything turn on well. Now you can access BIOS by pushing F2 and set the password to whatever you like.
I hope this article was anywhere near to help you. Much more detailed one can be found here, Or 
PDF made by same autor as previous web with more Dell models. I remind, the PDF is NOT my work.
Enjoy and good luck!
Picture 1. The chip containing CMOS, which has to be short-circuited is located under PCMCIA slot.
Picture 2. You have to short-circuit pins 3 and 6 while powering on the computer.
