A year ago, my cpu's fan wouldn't stop so I cleaned the cpu and everything worked fine. So today, my cpu's fan wouldn't stop and I thought I'd just clean the cpu like last time and everything would be ok. So I opened it up, cleared the dust off and then closed it up again. When I tried switching the pc on though, it wouldn't go on. There might have been this burnt smell when I tried to switch it on. I'm going to re-open the cpu in a minute...anyone know what could have happened? Thanks.

What do you mean the "fan wouldn't stop"? On *some* systems, the cooling fans will continue to run for a minute or two after the system is powered off. There's no problem, it's by design. As for your current problem, did you actually remove the HSF from the CPU? If yes, did you remove the old thermal material & then apply a fresh dab of paste? Are you sure the HSF is securely fastened & the fan is plugged into the correct header? Did you double check all your connections? And when you worked within the case, did you unplug the power cord & use the proper ESD precautions? "If my answers frighten you then you should cease asking scary questions" - Jules Winnfield (Samuel L. Jackson) in Pulp Fiction

Sorry...I meant the fan won't stop because the computer was constantly overheating. When that happened last year, I read online that all I needed to do was clean the dust off under the fan because the dust created this insulation layer keeping the heat inside (I don't know what that part of the motherboard is called).

I'm still not getting it. The fan's not supposed to stop. When you power on the system, the fan spins up & *should* stay running until you power off. Anyhow, I assume you're talking about the heatsink/fan (HSF) that sits on top of the CPU? If the CPU is overheating, it could be that the heatsink is dust clogged, but it you knocked it loose while cleaning it, it should be completely removed, thoroughly cleaned & a fresh dab of thermal paste applied. Here's the single core instructions: http://www.arcticsilver.com/pdf/app... And here's the dual core instructions: http://www.arcticsilver.com/pdf/app... "If my answers frighten you then you should cease asking scary questions" - Jules Winnfield (Samuel L. Jackson) in Pulp Fiction

My problem is no longer with the fan though, my problem now is nothing happens when I try to switch my desktop on (I'm on my laptop right now). :( I don't think I knocked over anything while I was cleaning the cpu, I was very careful. I really hope the problem isn't with the motherboard...is it possible that the power supply needs replacement? BTW, I'm very grateful for you help.

proper troubleshooting would keep your focus on the components you've been messing with, i.e. the CPU and heatsink/fan (HSF). Your CPU is a chip only slightly larger then postage stamp. Your HSF is a metal structure about the size of a baseball with many thin metal fins, and a fan that sits on top of the heat sink. It is still not clear if you removed the HSF from the motherboard, exposing the CPU, or if you just took the side off your case and removed dust from the exterior of the HSF. if i had to guess, i'd say that you probably did remove the HSF and then did not get it re-seated properly. which is why you "might" have smelled burning. which would mean you "might" have to buy a new CPU and motherboard. but leave your power supply alone.

I doubt you killed the cpu as the p4 is thermal protected although you may have shorted something on the board when you were in there. You could try another power supply and see what happens but Before doing that remove your cpu and ram and reseat them, you dont state how much ram you have but if you have more than one chip try one at a time. Remove all add in cards except for your vid card and unplug all your drives. Turn the computer on and see what happens. If the computer boots add one component at a time and see where your problem starts. It is quite possible you bumped the cpu heatsink fan hard enough to partially unseat the cpu.