Macbook (2.1Ghz white 2008 model) display won't turn on during start-up

Question

       hi all,
my problem is pretty simple.
Sometimes, after having used an external display with my Macbook (using the Apple Mini DVI > DVI adaptor: http://store.apple.com/au/product/M9321G/B?mco=MTY3ODQ5OTY ) the Macbook's own display won't power on during the next boot-up. I can hear the hard-disk start up, and the system boot, and I'm even able to navigate my way into the user login and shut the computer down (all via the keyboard). So the computer is definitely powering up (and in a seemingly normal way), but the display isn't.
For example, last night I was using an external display with my Macbook. This morning I plug the display back into my Mac Pro, then power-up my Macbook. But the Macbook's display won't turn on. It's as though it thinks the external display is still plugged-in. I know, that if I re-connect the external display, the Macbook's login page is displayed, but only on the external screen.
The problem only arose after I started using this particular external display (a cheap 22" BenQ, model G2222HDL 'senseye 3'). I wouldn't normally use the display with my Macbook, but my Mac Pro has a **really** noisy optical drive (CD/DVD), which makes watching films pretty unbearable (if you're into sound anyway, which I am).... so until I get my Mac Pro's drive sorted (when I have a few spare days when I won't be needing it), I'll have to continue using my external display with my Macbook to watch films.
My Macbook's specs are as follows:
2.1Ghz Intel Core 2 Duo Macbook (2008 model - I think, maybe 2009?)
4Gb RAM
Running OS X 10.5.8
Are there any known display issues with these models of Macbook? Or are there any software/firmware updates that address any display issues?
In general, I don't update the software/firmware unless I need to, or unless something doesn't seem to be working properly.
However, I've just checked Software Update, and noticed there is an SMC firmware update (1.4). So I might give that a go and see if the problem persists.
However, the problem is intermittant, and kind of unpredicatable, so I might not know for a while if it solves the problem or not.
Has anyone else had a similar problems?
Alternatively, is there a possibility there is some kind of virus or file-corruption causing this problem?
My girlfriend has a similar model of Macbook (which she's never had any problems with). Recently I moved a whole bunch of files from my Macbook to hers, and she noticed that hers had a similar issue starting up, where the display failed to turn on.... so it seems like
a) it might be a common-ish problem in this model of Macbook
or
b) perhaps I've got some kind of virus on mine?
I've been using Macs for about 15 years (yet, I don't consider myself a computer expert, when it comes to the technical details). In those 15 years, viruses have never really been a concern to me, as I've never really thought they were much threat to Macs. However, I guess, as Macs get increasingly more popular, there's a possibility people will write more Mac-specific viruses...?
Anyway, I'm open to suggestions : )
Regards,
Sam
PS I've tried:
-re-starting the computer multiple times (which sometimes works, but usually doesn't).
-re-setting the SMC (which usually works, but not always)
-plugging/un-plugging the external display to try and jog the computer into noticing it isn't connected (which doesn't work)

Answer 1

let me ellaborate on this part:
"b) perhaps I've got some kind of virus on mine?"
I mean, " perhaps I've got some kind of virus on mine, and I've passed it on to hers?"