Mercury 845G Strange Problem

I get this problem randomly. Sometimes, it causes the problem. Sometimes, it works fine.

The problem is that when I turn on the CPU, my monitor does not show anything. I think it is not booting. I have checked all VGA things and monitor and its connection with the motherboard are OK.

Can anybody help to find the reason?

Going to need some more info if you want us to accurately diagnose your issue.

-Does the monitor eventually come on, or do you have to power cycle in order for it to respond?
-Are you running onboard video, or a slot based video card?
-Have you updated your video drivers at all recently?
-How are you connecting to the monitor? DVI? VGA?
-What kind of monitor and how old is it?
-What are your general system specifications? CPU, RAM, PSU, HDD, etc...

- Power cycle is not needed
- On Board Video
- How to upgrade the video drivers at the boot leverl? I think it is not needed because sometimes, it is booting without any problem.
- VGA connection
- 15'' CRT Samsumg Monitor. 4 years old.
- Intel Celeron 2.4 GHz. 256 KB Cache, 512 MB Ram, 40 GB Hard drive, Windows XP Professional SP 2

Since you're running onboard video, the drivers are on your motherboards chipset do the work there.
Make sure the connection is clean for the VGA connection(No dust)
Does your monitor make a clicking sound when it boots, and then clicks again before it actually comes on?
From the sound of your system specs it sounds like that system is a good 5 years old; and sadly that's usually going to be the base point age wise for electrical issues. Could be the Onboard NIC going out(happened to me and a few other PCs I've worked on around the 4 year mark), DIMM slot, or anything really. So it very well could be a warning sign; or it could be the monitors relay tripping it. Which could be a short, or the power supply going into protection mode. Have you tried another monitor by chance?