VAIO TXN19P not keeping time. Maybe Cmos battery??? NOT SURE.

SOURCE: Cmos battery

You must enter the laptop internally that is.  In some laptops it is soldered to the motherboard be prepared to find that.  Did you know it takes a long time for the battery to be stuffed because.  Are you certain that the problem is the battery?.  If it is stuffed your bios would have resetted itself to the default settings and will not hold your settings.

SOURCE: CMOS Battery

you want to be looking for the rtc(real time clock) which is a little unit not much bigger than ur thumb nail.next to it youll find the battery(sometimes heat sealed with plastic.)if you remove this battery for about 5 mins then replace it or in ur case maybe renew it this will reset bios to factory(also method for bypass passwords as every thing is reset)
you might also wanna check ur video card.(connections and dust)

SOURCE: SONY VAIO CMOS BATTERY LOCATION

Correction...

The CMOS battery is located under the system board, and is not accessible from under the keyboard.

The keyboard has to be removed in order to change the CMOS batter, (another reason why I don't like Sony laptops.)

Below is the batter I just pulled from a user's VGN-FS660\W

SOURCE: Cmos battery on a Sony Vaio Laptop VGN FE31M

It should be located somewhere on the motherboard wich means pulling the computer completely apart which unless you know what you are doing may be a little difficult for some, but not inpossible.

SOURCE: How Replace CMOS Battery in Sony Vaio VGN-FS742 / PCG-7D2L?

Here is are disassemble instructions for you,

Good Luck