You can buy the batteries from most Big Stores nowadays or any half decent electrical shop.
From £1 - £3 there all the same CR32 If i remember.
Confirm its the battery?
1. Set the time and date correctly.
2. Shut down the laptop.
3. Remove the Main Charable batery.
4. Leave it over night.
Morning.
Power up the laptop.
Press DEL or what ever it is on a Dell for the BIOS
Navigate to Time and Date.
Confirm its the correct time.
If its not then the battery is at fault.
If its ok continue to boot into windows
as it may be software changing the time and date !!
Hope that helps.
Sometime the CMOS battery can be accessed in the battery compartment but not in this case