Computer starts up, Monitor stays blank..Suspect:Motherboard

Okay I have a pretty good idea of this one already but would like second opinions to go over what i figure and point out any flaws in my logic or thoughts

My father-in-law's recently removed a hard drive from his computer. This harddrive was hooked up to the Power but not actually hooked into the IDE Chain. So the way I see it to all extents and purposes it was never there(apart from to the power supply). Now when he starts his computer the monitor has a message "Check Signal" on it, before it goes into Standby mode. The CPU light will keep lit up for a short while then go off as it boots into windows, no beeps on startup from the CMOS POST. I also placed the XP setup disk in the CD-ROM drive to check whether the hard drive had become corrupted somehow and was failing to boot up, the CD did not spin up at all and I believe its #3 in the boot order. Swapped monitors and still no picture, so I believe it comes down to one of two things. The motherboard(it has on board video) or the Power Supply. The PSU can be ruled out by placing a different PSU in which I intend to do. I don't think it is the PSU though as if it wasn't supplying the Motherboard correctly I feel it would not even boot up or would get part way through the boot process before locking up. Now can anyone think of anything else I might have missed? I would be glad of any opinions or thoughts on the issues.

Another note is this also occured a few months back, it didn't work for a week and I didn't have time to work on it, when I went to it started working again for no reason that I could tell, its been working fine now for a couple of months..

In case you are interested in more details his specs are:

PIII 450MHz, 128Mb RAM, 10G Maxtor HDD, Intel SR440BX motherboard with Onboard sound and Video.

Thanks in advance People,

Alkatr0z

try plugging the monitor in the other monitor plug. it may work if you are running the monitor off the onboard video card.

Have you checked all of the jumpers ect on the motherboard ? Check signal sounds like the monitor itself telling you the signal isn't getting through.

A jumper to select between onboard & pci/agp card may have accidently been knocked off or somthing along thoose lines when the HDD was removed.

Just removing somthing from the power supply shouldn't mess anything up like that

Okay I've solved the problem. As it turned out it was the RAM. I viewed a page on Kingston's website that talked about problems such as these and replaced the RAM with a handy stick and booted up with no trouble. The RAM was something I had previously discounted as unlikely as the computer appeared to be booting up into Windows XP without any trouble and I felt windows xp would certainly give 128Mb of RAM a run for its money. Important lesson to myself. Never discount anything that you consider unlikely as you never know until you test it! In case anyone is interested in having a look at the page I looked at here it is:
http://www.kingston.com/tools/umg/umg08.asp

Thank you all for your help, the idea about the jumpers was something I was going to test as well though curiously enough it doesn't seem to have an AGP slot which really reduces the ability to upgrade video cards, though a CPU upgrade would be in line with that and would bring a new motherboard anyway.

Alkatr0z