Computer won't boot.

Hello there. Okay, so the other day I decided it was about time to clean all the dust out of my computer again. While I was there I decided to re-apply some thermal paste to my CPU too. I've done this many times and never had any problems but now my computer will not boot. Power is there i.e HDD and fans are spinning, mobo LED is on as is the DVD drive but no image on my monitor and no HDD activity (as far as can tell from the HDD activity LED). Now, I have a feeling that it may be my graphics card as I was a bit heavy handed while cleaning the dust off of its fan and now when I power on, the fan on the card sounds like it's running at double speed (or the bearings are gone). I've tried to boot various times and the one time it did boot the graphics card sounded normal but the system completely froze after about 20 mins. I've tried removing my RAM to see if that's the problem but the thing is I have no internal speaker so I can't even diagnose it from the beeps.

Anyway, if it was the graphics card that was dead, shouldn't I still be getting an image from the onboard graphics ? I'm using an ASUS P5LD2-SE which does not have a built in graphics card but it should still be able to display the bios shouldn't it ? Any help will be appreciated, computer specs are as follows:

ASUS P5LD2-SE
Core 2 duo 2.4
Geforce 7800 GTX
2 x 1GB DDR2 RAM
250GB SATA HD