have u tried removing the battery and only have it plugged into an outlet and use that backup power then try???

ive had problems like that before with computers ive worked on over the yrs and had to do it this way cause the battery was soo far gone it wouldn't even work anymore...

The CMOS Checksum Bad Error is an error that occurs when the CMOS values are incorrect. Usually to guard your Bios software, your CMOS memory stores a specific value, each time you boot up your computer this value which is a number is checked against the stored value in the CMOS memory. If these two values are different, it causes an error message. If these values are the same, the computer proceeds as normal.

Each computer usually deals with the CMOS Checksum Bad Error differently. Some computers will warn the end user and continue to boot up using settings in the CMOS, other computers might warn the end user and then use the default settings in the BIOS as the correct settings and carry on with a normal boot or reboot. Usually, within the error message it will state which strategy your computer has employed.


Bad RTC battery
Bad Checksum (CMOS)
Check system, then press [F1] key

The Real Time Clock (RTC) battery provides power for the internal clock/calendar and for maintaining system configuration settings.

This error can occur when a machine has been left turned-off for an extended period of time (approximately one month), and is the result of a depleted RTC battery. Other symptoms include not being able to execute Windows Update properly, errors/problems executing application software (example, Norton Antivirus), and the appearance of the Windows XP "Desktop Clean Wizard".

Resolution:
This condition is easy to fix in 3 basic steps:

1. Reset the BIOS defaults
2. Set the correct date and time in Windows.
3. Charge the RTC battery


See full detail here-

http://www.tech-faq.com/cmos-checksum-bad-error.shtml

do you have a boot menu like "hit F12 for boot menu" on the 1st screen

if you burned the CD as an image as suggested use the boot menu if you have one

and replace the battery as soon as possible

the cmos battery is the one you need to change as said its a coin type battery and very rarely is it a chargeable type a new one costs cents and is simple to change