First take note of whether you are losing time while the computer
is running or while it's powered off. If it's while powered off,
the problem *is* very likely the battery. But if it's while
running, it can *not* be the battery.
If the clock loses time while running, try this:
Open a command prompt window (Start | Run | cmd) and enter the
following commands:
net stop w32time
w32tm /unregister
w32tm /register
net start w32time