I would like to fix my old desk top

Hello and Welcome to the Webroot Community Forums!
Could you please tell us more about your problem?

Depending on the type of desk top, and the problem, you may or may not be able to fix it.  At a certain point it is no longer worth the investment unless you just love your old desk top computer.   What are your plans?   You may be better off buying a kit and building from scratch if you just want the satisfaction of building a system.  You may or may not be able to use old parts.