Problem with Computer, Windows XP Operating

Hello,
I had a computer that messed up over 3 years ago. I had the hard drive, power supply, fan, motherboard and xtra memory installed into it. It show a blue screen when I try to start it up. I had someone look at it and they told me that I needed to have Windows XP opertating system loaded onto it. They were going to charge me $100. I don't have the spare money nor do I have the recovery disk. Can you help me or tell me how I can load the operating system on to it? Could I buy Windows and install or what do I need to do? Thanks for all of your help!
Cindy