I do medical transcription. I have a desktop that is my workhorse. It is a Dell Optiplex 745 running Windows XP Home edition. Jewelry making is for fun; MT pays the bills.
The short version solution: Check the voltage switch on the power supply at the rear of the computer. Make sure it's set to your household current, which for us was 115, not the 230 it was set to as a default.
Here's the story.
About a month ago the computer just died. It had worked fine the night before, but when I got up the next morning, the power switch had a blinking amber light behind it. I thought it was in hybernate mode, so I pressed and held the power switch for 5 seconds. Nothing. Blinking amber light. So I completely unplugged the computer from the power and let it sit. Finally the amber light went out. I plugged it back in and hit the power switch. Nothing. Bad, bad news.
I got on my netbook and started googling. Turns out, this was a symptom of either a failing power supply or a failing motherboard. I looked inside the computer and could see that the small LED light on the motherboard was glowing green (a good sign), so that likely wasn't the problem. The diagnostic also said to unplug absolutely everything from all the USB ports except for the monitor, the mouse, and the keyboard, and then try powering up.
I did that, and the computer came on. Bingo! Power supply. While I had it up, I hurriedly transferred my files to my netbook for work, then shut it down. I ordered a new power supply off eBay from DFW_parts_depot. They had exactly what I needed (link to their shop here: http://myworld.ebay.com/dfw_parts_depot/). They got the part to me the next day.
Finally, a couple of days later, when I had time to open the computer, we switched out the power supply and hit power. All lights turned green, and everything started coming up (yahoo!)... but then it got to one driver and wouldn't boot up.
I tried everything. I rebooted, rebooted, and rebooted. I got on the internet and Googled. I saw all kinds of entries... everything from saying that the file was corrupt to the fact that the power supply was bad (bad? I just bought this brand new!), on and on. I tried for a couple of hours to find the problem.
Finally, I was reading a tech forum, and it said to check the number lights on the front of the computer (1, 2, 3, 4 right above the power switch; most people don't notice). I did that and saw a sequence of numbers that indicated low voltage.
For some reason, at that moment, the light bulb went on over MY head. Check the voltage switch on the power supply itself!! I did. Lo and behold, it was set to the wrong setting.
I took a nail file, flipped it to the right one, prayed, pressed the power switch, and BINGO!! Everything booted up just fine.
So my advice is this: If you have this problem and you have somehow moved your computer to vacuum behind it or if you've moved it, replaced the power supply, or whatever, if you get an mup.sys boot error, check the voltage selector on the back of the power supply.