very strange!!!! blank screen on start up

My PC had been on all evening, it was just idle, when I noticed a blue error screen, unfortunately I didn’t note down the error number. Anyway I reset but wouldn't boot up. Reset again this time going into the bios to ensure everything as it should be which it was, exited but wouldn't boot. Reset went into bios again, and as motherboard has dual bios went into the backup bios but before I could click on the boot up with backup bios, the cursor started moving by itself and screen went blank. Reset again this time no beep, no screen.

System details

Gigabyte SINXP1394 - motherboard
Pentium 4 Intel 2.6Gz CPU
Sapphire Radeon 9200 128MB AGP graphics card
one stick 512MB DDR memory
160 GB maxtor hard drive
15 GB hard drive (this is C: drive) - think its a FUJI make
windows XP professional - SP2

Opened up case everything seemed in order no loose cables no strange smells etc. but quite dusty. Completely stripped everything down and gave a good clean. Also removed the CMOS battery to reset the BIOS. Now the inside of my case was spick and spam, decided to give it a try. Only attached the basics - CPU, RAM, video card, the 15GB hard drive (c:drive), monitor, mouse and keyboard. Power on (beep) and screen comes on. Success I think. It says something about a new processor being installed, and prompts me to update the BIOS which I do. Exit bios boot up, select boot up in safe mode, which is successful, restart this time boots up window in normal mode, everything seems fine.

BUT.

Disconnect from power connect the other hard drive, start up again, no beep and blank screen (led light on screen stays orange). I disconnect from power and go back a stage disconnecting this hard drive so i'm at the stage where I had previous success, but again nothing happens - no beep, no screen.

I thought at first it might be either a blown CPU, a blown motherboard or a blown memory. But what I cannot understand is that it didn’t work then I cleaned everything then it worked, and now it doesn’t again.

I've also tried an excellent troubleshooting guide (originally posted by cody, and re-posted by dohray) you can read it at http://www.cybertechhelp.com/forums/...ad.php?t=20019
2nd item down. basically when I got to step two, there were no beeps which would indicate my mobo is faulty, but if that is the case why did it work after I cleaned everything, and reset the bios.

PLEASE HELP !!!!