Standbye Issues...Does not resume

Hello,
guys i need help.....i have a compaq machine with windows xp media centre and recently i had the motherboared replaced because it burnt or sumthing. now to save energy i have been trying to activate the standbye mode which apperently switches off the computer but can be resumed on keyboared or mouse touch. Iv tried this and when the computer goes to standbye mode it remains off. the power light flashes on the computer and the monitor turns off. the only way to turn it back on is by pressing the power button which restarts the entire system

am i doing something wrong heeeeeelp

kind regards