Bios system clock stops wen turned off and on boot error 0251:system cmos checksum bad message comes up

that seems to have solved the problem with clock thank you so much. but now i have another problem with booting. i can not boot from the power button so what i have to do is switch it off from the mains socket and switch it back on in order to boot without pressing power button. any ideas? will be much appriciated... that seems to have solved the problem with clock thank you so much. but now i have another problem with booting. i can not boot from the power button so what i have to do is switch it off from the mains socket and switch it back on in order to boot without pressing power button. any ideas? will be much appriciated...

my acer travelmate 250 won't start normaly untill i conect the power pack then it works without i press on power button and cmos error apear when i enter the bios and load the default values and save and exit my laptob work but when i shut it down it won't start never untill i remove the cmos battrey and remove the power pack and reconect it again then my laptob start working without i press on the power button etc............. my acer travelmate 250 won't start normaly untill i conect the power pack then it works without i press on power button and cmos error apear when i enter the bios and load the default values and save and exit my laptob work but when i shut it down it won't start never untill i remove the cmos battrey and remove the power pack and reconect it again then my laptob start working without i press on the power button etc.............