computer cannot start or boot

My PC won't start either! I was trying to fit/install a new USB2 + 1394 firewire card and everytime Windows XP started and found new hardware, my PC stopped responding everytime it got to searching for drivers......

I couldn't even use alt+ctrl+del to stop the process or restart my PC, so I had no option but to force it to shut down by pressing the Power button until it went off.

I then removed the USB2+firewire card from the PCI slot and tried to start my PC. But the power button is not working at all!

Have I killed my PC completely? One member suggested pulling all the cables except ? (can't remember right now but I'll look back if necessary). I don't quite know what he meant by pulling all thr cables? What cables? Where? How? How do I pull the cable for my sound card? Or should I remove that from the PCI slot completely?

Another member suggested replacing the power switch. But how do I do that, if that is the problem?

I'm so upset that I might have caused a serious problem with my PC, that I doubt if I'll be able to sleep until such time as I can get it fixed. And as I haven't the money to call out an engineer, I'm hoping that someone might guide me through whatever steps I need to take? PLEASE HELP?

Good amount of starting problems here is mine. But mine did not shut down like these have are.
I did have one that did the same exact same thing as yours it would turn off after a second or it would almost boot up Windows and then shut down and then at it would boot up OK. I kept pressing the power button until the computer started. I ended getting a new computer.

Ok on the computer I am using now here's what happened.

My computer shutdown last night because the electric went off for a while. No storms were around. Anyway I put the omputer on this morning and saw the screen just flickering. So I shut the computer back off by pressing the power button on the front of the computer. I. It noticed the screen did not display the wording that usually shown up before Windows starts. So first I unplugged my USB external hard drive rebooted computer still the same. Then I unplugged the keyboard then there was wording on the screen. But it listed the bios and the fact the keyboard was faulty. Ok plugged in the keyboard and decide to unplug my second external hard drive from its power source and plug it back in. Just guessing things. Power up the computer still. I words on the screen but the Windows version choices showed up and the computer booted up Windows fine. Now the computer is working OK.

I also noticed before I had fixed the problem the 3inch floppy drive letter stayed on.

So the question is. Any idea why this would have happened?