Every time I start up this computer I get the messages,
CMOS checksum error - Defaults loaded
CMOS battery failed
Press F1 to continue, DEL to enter setup.

I know that the motherboard battery is on its way out, but how long will it last?
I looked at the mptherboard to see the type of battery but could not see one. The only thing the looks as if it is the battery is a long fat rectangle with the words "Houston Tech HT12888A Real Time 9639A116669V" on it.
Is this the battery? and if so can you still get tham as it is an old machine (Pentium 166).