I have had the lap top (HP COMPAQ 6190p) about 4 years and all of a sudden it is asking for a product key? I cannot locate this. I am unable to get onto any of my users as well.

Product Key for what?

I think that happened to me once and I had to check it from under the battery after I removed it. Anyway I`m not sure, but what I do know is you should just contact HP customer service and ask them.

Again, this depends on what product key you are needing. If it's for an OS and it's a genuine copy of Windows you can find it in many places, (registry, tag, etc.. )