On Tuesday, November 13, 2012 2:35:37 PM UTC-8, wrote:
[dead CMOS battery]
I WILL open it, but is there any chance I can find the Gateway2000 P5-75
schematics and battery specs online before I do so I can put it all back the same day?

Gateway, P5-75, and probably Gateway 2000, are all ambiguous; you cannot
really tell what the motherboard uses without an exact model number
of the (probably Intel) motherboard.

What you CAN do, is get to Radio Shack and buy a coin cell (CR2032) and
an alkaline 4.5v (#840) and maybe a 1/2AA (#5150) lithium cell, and
after you see the innards and plop in the battery you need, take
the others back to the store for a refund. For ten-year-old hardware,
those three possibilities cover almost every desktop computer.

Two caveats: if the old battery is leaking corrosive goo, you will have
to clean that up. And, the replacement sometimes does NOT LOOK LIKE the
original battery (look for a labeled socket that fits a pigtail on the alkaline
battery, and use the handy velcro on the battery to attach it ... somwhere).