Hi. I built my computer about a year or two ago and have slowly been upgrading parts. The most recent of addition parts have been in place for at least 6 months. I run an updated version of Vista Ultimate with all avilible service packs. Today a few of my broswers were crashing on me so I decided it would be a good time to clean the dust out of my pc. The only tools I used were cotton q-tips with no liquids whatsoever. I did the following things in the exact order specified. First took out the cpu heatsink and cleaned all the dust out of it, then i removed the cpu and cleaned up the outline of dust it left on the motherboard. I carefully inspected the bottom side of the cpu and the pins on the motherboard facing up to makecontact with the many small circles on the bottom of my cpu. Everything seemed to be in order and I didn't notice any bent or discoloration of the pins or the cpu itself. I then removed both sticks of dual channel ram I have and dusted those off. I finally removed my pci-e graphics card and dusted it clean. I noticed plenty of dust in between the the card's heatsink and chip so I removed the heatsink and very carefully cotton swabbed the surface of the heatsink and around the chip. I then took q-tips to the motherboard cleaning up excess dust. After putting everything back together properly and exactly the same way I took them apart, I tried booting. Normally my computer (presumably the mobo) makes a few beeps just as it is powered on (to which I have come to understand as post) but instead there was none. I also noticed none of the video outputs were working. I have two monitors plugged into the one video card and none of them were receiving any sort of signal from the card (ie instead of the monitor's power light going green they both simply stayed at the flashing orange indicating standby). After freaking out for a bit I tried reseting the cmos. there were three cmos pins; my machine only powers on when the jumper is in "normal" mode and will not respond to the power button at all when in "clear" mode. I also tried removing the lithium battery on the mobo for 10 minutes and reinserting it but to no avail. My system just keeps doing the same thing; powering on puts all of the systems, cpu's, and graphic card's fans in motion, and powers the hard drives. The mouse and keyboard lights came on and stayed on the first few times I tried booting but now and after disconnecting and reconnecting them they only light up for a second at the very start of a boot and then turn off. I have tried almost everything else I can possibly think of and read in forums online and I have yet to find a cure for this problem. I have also tried removing the video card and pluging a monitor into the onboard video output before and after an attempt at cleaing the cmos, still nothing. What bothers me is how it seemed to be working fine before I began dusting. Below I have listed my system specs:

LGA 775 mobo with 2.3gz Dual Core P4
2 dual channel sticks of 512 ddr2 ram
pci-e 16x 256mb geforce graphics card
500w power supply

One thing that was fishy is that every once in a while (about once a month) my pc would shut itself down during intensive graphical or processor heavy use. A simple flick of the power supply from on to off and then back on again would fix this problem immediately though. If you have any suggestions please let me know as I am running out of ideas and it is urgent that I get back onto my computer and finish the work that I was in the middle of. Thanks in advance for any support.

You shouldn't have removed the cpu nor the heatsinks.

In any case, did you clean the old thermal paste off and re-apply a bit of NEW thermal paste before re-attaching the cpu and gpu heatsinks?

test your ram with memtest 86+

test your PSU with a tester tool AND a multimeter.

At first I did not reapply thermal paste but after reading a bit online I did remove and reapply thermal paste to both the cpu and and graphics card cpu. I do not have a multimeter or any way of using the memtest cdrom as I cannot get any boot screen or post beeps. The fact that there are no beeps leads me to believe there is a problem with my motherboard or power supply. If the problem was with my cpu, ram, or graphics card alone, it would at least beep to tell me so on boot wouldn't it?

I have since been trying to boot wouthout a hard disk attatched because I'm paranoid about that thing breaking somehow. In an attempt to reset the cmos I took out the lithium battery and shorted the "clr_cmos" pins with the power supply unplugged all night. After returning the battery and moving the jumper back to the normal cmos position I tried booting the computer to the same problems I was having before. I have not once heard post beepr or gotten video ever since trying to clean my pc.

Take out the graphics card & RAM and then power on, do you get any beeps?
Did you plug the speaker back in correctly?
Try re-seating the ATX & 4 pin power connector.

If none of the above work, re-seat the CPU and inspect the CPU socket carefully to check for any pins out of place. DON'T touch the pins.

If none of the above works I'd suspect your PSU being dead because you mentioned it occasionally turning off.

What brands and model numbers of components do you have? Your description is a bit vague.

I have fixed the solution with a new psu and motherboard. I believe the psu was going anyway because my pc would sometimes just turn intself off. I must've shocked the board by using dry q-tips. Guess I learned my lesson about compressed air. Maybe I should be lucky nothing else was shorted out. Thanks for all your help.

compressed air FTW