BIOS Battery

you will basically have to dismantle the laptop which isnt a job for your average joe.
best you take it to a shop in this instance which is something i don't normally recommend, as most of the time its not hard to do. but in this case with the amount of connections that have to be removed and it being a complete disassembly. i feel i should recommend you take it to a pro.
but if you are comfy doing it and or capable then heres a guide.
http://forum.eeeuser.com/viewtopic.php?id=6036

you will see you need more than just a battery. as its in the blue heat shrink cover.

^+1 i agree disassembly and reassembly of laptops isn't easy for the uninitiated.
Lots of tricky/delicate connections.