. . you boot on the CD and when given the choice between Install and Recovery Console you take Install. Then it detects existing installations. The choice is "R"=Repair and Install. At this point you take "R". This reinstalls a lot of stuff but it tries to use all the drivers, desktop, etc. from the existing install.
I used it today after removing damaged networking and it worked fine. I think it does something worthwhile about 30% of the time.