I fixed it the last time, but it is now back again.
The symptoms are exactly the same and I have the same specs.
Old Topic can be found here:http://www.d-a-l.com/help/windows-vi...n-problem.html

I've tried everything recommended in that topic, everything that I found myself and it still wouldn't go away!

Also, I noticed that it happens because I started the computer with the monitor off.

I've rebooted, tried system restore, uninstalled and reinstalled both graphics chipset and monitor, tried the GMod thing too.

Any help would be great as I hate wrongly set screen resolutions...

Having the same problem come back is best handled in the same thread so that the people that already helped you will know that you've updated the thread & so that them (and others) will better know what is going on. Have you tried installing a new graphics card? I saw something about an nVidia graphics card; their drivers might have settings to force windows to display the monitor at the proper dimensions. Play around with that. You should try installing the drivers of that monitor so that Vista will know exactly what monitor you are using a what dimensions that it can use. You should have gotten a setup disk with the monitor if not download them from the HP website & be careful that you get the drivers correctly. Also if this monitor is new then you should be able to contact HP for live help.

Actually, I've fixed it already.
I'm not sure what I had done but it's fixed.
What happened and what I tried:
BSOD due to iTunes
Late at night so system reboot with monitor off
<Problem Started>
Reinstalling Driver
System Restore
GMod
Force Screen Resolution
Reinstall monitor driver
Edit registry(Setting every single 1600x900 value to 1440x900)
Restarted, didn't work
Tried editing again, late at night, didn't restart.
Turned Monitor Off
Morning BLACKOUT with monitor off.
Computer started with correct resolution...

So I now know that the problem is caused by turning on the system with the monitor off, but what fixed it?

Who knows but I've never heard of anyone doing a regedit to change the screen resolution. Messing with the registry is dangerous if you don't know you are doing. I don't do anything with the registry unless I've read about it. I hope you backed up the registry in case that backfires. Let us know.