HP monitor requires manual restart

Hi All,
I owned a HP w1907 lcd monitor. It comes as a package with HP Slim line S3520d Desktop Computer.
I have been using this for the past 2 years, and all is working properly.
Except yesterday when it was running so weird. Normally, whenever I turned the computer off or turned it on again, I don't need to press the monitor's power button at the top of it because it will automatically open when I open my CPU. But since yesterday, I have to run this process again and again. I don't know if this has settings for the monitor, for all I know this is a plug and play hardware that doesn't need any software to run.
The monitor specs are:

Model: HP w1907
Size: 19"
Type: Widescreen LCD Monitor with built-in speakers
I searched on Hewlett Packard website, but there is no available software for this. I want to know how I can change it from my original settings. The CPU specs are:
Processor: Core 2 Duo E4700
Chipset: NVIDIA Geforce 7100
Motherboard: Irvine
RAM: 4 GB DDR2
Hard Drive: 320GB SATA
Operating System: Windows XP Professional Service Pack 3
Please help me. I am also experiencing a horizontal green lines appearing sometimes.
Then after doing a reboot, the lines will be gone.
Can anybody state the cause of this problem and what can I do to fix this?
Thanks
