Unexpectedly low cpu speed

Hello I have a pc with a pentium 4 3.0 ghz cpu, 1,5 gb Ram ddr1 and windows xp service pack 3. I bought this pc 4 years ago. Last month i tried to turn the pc on and there wasn't any signal at the screen. When i tried to reset, it started at once but gave me the message "overclocking failed!press f1 to enter bios or f2 to load default values and contunue". Weird thing is, i didn't try any overclocking and i didn't find any change at the "overclocking" value in bios. So, I just quit and the pc continued loading, but the cpu speed was much lower than before. Every single program loads much slower since then, and at the ctrl-alt-del screen i see that it takes a big share from the cpu (firefox for example may use 100% of cpu to load a single page). Every time i turn the pc on since then, i experience the same problem. Some times,also, just resetting won't make the pc continue loading, but i should reset many times before it eventually beeps and shows me the "overclocking" message. I thought it was a power problem, but i was told that it may also be a problem with my motherboard. What should I do?

It could also be software or a virus, causing the problem. Try a system restore to two months ago and see if that helps.