compaq presario M2000

my laptop is DEAD (compaq presario M2000), power pack works fine (been tested) main battery is shot won't hold charge at all so been powering it by mains (soon as I unplug it it shuts down instantly)
This laptop was working fine (a bit slow but I can't reset it to factory settings as it didn't come with a restart disk and this is the only model in the world that you can't F11 or whatever it is when it starts up to reset it)
one day I was using it and it shut down and wouldn't turn on I left it for about 5 mins and then it turned on fine but one day I went to it and nothing no power no lights not even for a split second, I read on tinternet it's the motherboard then I read it could be the CMOS battery and taking this out and leaving it out might work but after Reading more I don't know if it is anything to do with it
but just thought I would ask if any one knew if this was anything to do with it, I was told it could be just a build up of dust on the fan (told this when I said it just shut down and after a while it turned on) I took it apart and cleared off the fan (wasn't that dusty) had a look and I can't see any burn marks or any loose solder joints, I also checked the CMOS battery and this had a current.
any help or any ideas what's wrong And what I need to do (needs to be cheap)
thank you for Reading