Recently we've been getting a lot of questions from people who basically have the same problem - they've assembled a PC but it won't start up correctly!
There is a standard troubleshooting procedure for that scenario which everyone should follow, and I'll describe it to you in a moment. But first let me address a few common issues for people whose older PC won't start any more:
* Does it 'hang' when it's loading Windows or when it reaches the desktop?
If so your problem is most likely a software one and NOT a hardware problem. You should, of course, check that all the cooling fans are working (especially the ones on the processor and the video card) in case overheating is a problem. If you have two or more sticks of RAM which are from different manufacturers you could try using them one at a time in case a RAM mismatch is causing the problem. But other than that the only real way to see if it's a software or a hardware problem is to try running it with a fresh, clean installation of Windows.
Go buy a second hard drive. Extra storage space is always worth having and they're quite inexpensive. Swap your current drive out, put the new one in, and load Windows onto it. If all runs fine you know for certain that the problem is in your original Windows installation. If not you've got a second hard drive to use once …