Welcome to the CH forums.
There are many sites on the WWW which can explain how to format a hard drive and reinstall an Operating System,
here is just one of them. As pointed out on this site, formatting and reinstalling should not be considered lightly.
However, in order to assist you, you must give us information about your pc both hardware and software. Please read
this. Do you have a full genuine Operating System cd with its unique Product Key?
You must prepare well to do a complete reinstall without problems.