There are a couple of ways to try and reset the password. The first thing you can try is to remove the CMOS battery and leave it out for about 20 min and then put it back in. This will clear all the settings and should clear the password too. You will most likely have to reset the system time etc in the BIOS.
Another option is to see if there is a generic password for your brand of computer that you can use to get into the settings and clear the password. Many companies use a standard password just in case something like this happens.
If you can find the motherboard manual then there might be a jumper you can short to clear the BIOS password.
It sounds as though your motherboard CMOS battery is dying. When you are prompted to go into setup at boot up it usually means there is a configuration problem. One thing you can try is to go into setup and check the date and time in the BIOS and change it if it's wrong. Then save and exit and see if that solves your problem.
Once you fix the configuration problem in the BIOS then that message should go away. If your time continues to be off then you should change the CMOS battery. They are only a couple of dollars and you can get them at an electronics store or even Amazon. Just be sure to bring the old one in to make sure you get the right one.