Sounds like the warning is telling you to replace the RTC (real Time Clock) battery.
This part of the machine also holds the memory in your Bios.
Ok now we have this said it is easy to replace this battery with a new one.
With power off and why not unplug the power. Turn the computer on its side plugs on the left usually. Unscrew the 3 screws, usually, holding this side cover and slide it out a bit towards the plugs end just a bit, the cover will lift off. It is loose so should lift off. Has been a few years since I opened this case but you get the idea.
Now you see into the guts of the machine and also see all the dust and cat hair probably. Look at the motherboard and see there is a shiny round wafer battery in a socket. Carefully pry the battery out with (your fingernail) or a small device and be careful to just work on the battery not to scratch any part of the motherboard. The battery is held in by the silver part of the battery holder like a spring. The battery is in place under a lip so from the opposite of the silver spring. Push the battery sideways against this and flip it out from under the socket lip.
Oh yes now you are the wizard of the RTC battery.
Take the battery to the computer store or your choice and hand it to the clerk to ask for another new battery. And while you are there get a can of dust off, or two.
When you are back with the battery and dust off, carefully take the case outside and use the cans of dust off to blow out your fans and power supply and you may want to hold a face cloth over your nose. Any glasses will do even sun glasses just to keep the dust out of your wonderful peepers.
Now back inside you are ready to put the battery back in. If you touch the battery with your fingers it leaves oil on the battery which will in time ruin it. Small piece of paper towel, wipe the battery and head into the socket flat side down. Slip the battery on top of the socket and first dip it in at the silver spring and it will fall into the socket and be happy there.
Put the cover back on and now you have to boot the computer to the set up screen and set the time and date. Other settings in the bios can render your computer brain dead so be careful. It is a good idea to make sure you set the order of run to be DVD then hard drive.
Now when you boot up after saving your settings before leaving you should be ok. Windows will most of the time not boot with out the correct date, it checks this on boot from the last good boot file.
If you are running OK then you are the hero of the neighbourhood. If there is some trouble go to a friend and look up the manual for the computer using the model number or even the system number if you look, it is posted on the case either inside the side sort of handles or even under the bottom cooling fins on the front. In the manual you will find how to set the BIOS towards the end of the manual.