I have had a look at some threads and stickies on here, and now I am slightly more worried than when I started looking round for options. I'll explain why in a bit.

My PC (which I bought second hand, and was assembled by an individual) won't start up. It was fine when I went away one weekend, and I got back, and it stopped working. To give a bit more info, when I turn on the power, the red LED on the motherboard (an Asus P428X) comes on & stays on, and a green light on the front of the case which marks out a cylinder (and which I'm guessing represents the hard drive) flickers. The lights on the keyboard also flicker. Nothing comes up on the monitor though, and there's no whirring of the hard drive or grinding noise of the floppy drive.

I haven't changed any components recently, the last thing that was changed was that I bought some RAM over 2 months ago and the machine still worked fine till now. So my simple question is, what things do I have to check/tweak/test to find out what has gone wrong? Also & this might not be important, for a while now, whenever I turn on the computer from the wall (and even now when it's not starting up), I hear a sound like you get from a television when it's just been turned on, before the picture comes up. I have only heard this sound with my computer and that's why I mention it here.

And finally, the bit that had me worried, my room is carpeted, and after Catweazle's guidelines, I'm worried about how I CAN work on the inside of my computer. The only places that aren't carpeted in my house are the kitchen and bathroom!

OK so that's all the info I've got, I welcome all suggestions and references wth much appreciation.

The thing i would test straight away is that the CPU is not overheating and has burned out. Check around the large heatsink (usually with a large fan on top). If the chip underneath the heatsink looks damaged or slightly burned that is your problem there. If your problem is not that check that every cable connects fully to the right place. Often when changing some components, cables can get in the way so you may have unplugged a fan and not plugged it back in, possible relating back to an overheat issue. So my advice would be check every cable that it is connected to the correct place in case any have fallen out. Also turn on your monitor. Does the monitor display a "no signal" related message when the computer is off and the monitor is on? If not that may point to a monitor/GFX issue.
I would check what i have said first, then post back with what you have found out.
There is nothing wrong with putting a computer on a carpet as long as the computer case is grounded, i.e. connected something that has no charge. Putting a computer onto a wooden table/desk would be the best because it is at eye level and won't hurt your back when bending over it on the floor.
Hope that helps!