Assuming you are going to install windows XP or newer versions of windows what you need is
1º Your CD/DVD with windows.
2º Check your boot priorities on your mobo, so you can start from the CD and install.
Look in your HDD and save everything that matters to you, documents photos videos, whateaver.
Search the internet for the newest drivers for your computer and save them so you can install them on the first windows boot.
Once everythink is ok, insert CD, restart and then it will ask you to press any key to start installing the new OS, press and go through the options it will ask you if you want to format, say yes and continue.
Is easy go with the flow, I agree, next , next, next, I agree.........
And after 30 min you should have your computer running.