Just installed windows xp 3 days ago,
Now I put on my PC and it won't start... the lights donot even come on
I changed the cords but it still won't work

First - check the simple things, does the plug socket still work etc. I know it sounds stupid but it normally turns out to be the simplest things you don't normally consider.

You said no lights come on but is there any noise? Fans spinning that kind of thing.

If the answer is no, I would say that your power supply has blown. This is a reasonably simple repair if you are confident working inside a PC. If you are not though - take it to a PC repair shop. Mucking around in a PC can be dangerous for both you and your machine. It shouldn't cost much to get repaired.

Power supply's can blow for a number of reasons. If your electricity supply has a power surge and your PC isn't plugged into a surge protector, that can do it. Also if your PC is connected to the phone line and that phone line get hits by lightning during a storm, that can also blow the power supply (happened to me before).

Well for me, when this has happened, it usually just something stupid like my memory isn't plugged in all the way.

What it could be more likely is that if you seated your motherboard yourself, you seated it incorrectly. If metal is touching the motherboard on any point other than the points its supposed to, it will cause a short, and the computer will not start. This seems to have been the most common problem with me.

Other than that, more often than not, the motherboard will be dead, or even the CPU. Bad PSUs are also a problem, but seem to have been less common for me.

I just had a problem my computer wouldn't boot because there motherboard was bad! You seriously have to try everything you can think of, even the silliest things. I have spent hours fixing and repairing computers.

But like Theo said, if you aren't comfortable going through a computer, then maybe you should take it to a repair shop