Need to reinstall main hard drive

A few months ago I received a used Mac Pro (cheese grater). I edit music and the older Mac Pros do a very good job.

I have a small problem - when the tech set up the computer, he asked for my user name and password so he could install. When the computer arrived, I migrated from my old iMac. The problem is that I used the same user name and password when migrating. So after migrating, I realized that the computer had created another user with my name with a 1 after it.

Because of this, I had to create another "documents" and "downloads" link in the side bar. Not too much of a hassle. However, I believe that having the extra admin account has cause some anomalies with my music editing software.

I would like to reinstall everything on the computer without losing data. I know there is a way of doing this, but I'd like some advice on how to go about it.

Thanks!