Suggest you check out the Ebay auction site - they have a current bid of £10 on SE upgrade CD.
Certainly, do not try to install over the top. You are better off refpormatting and restarting - gets rid of all that chaff on the system. You will need to insert your old CD at some stage during the process. Alternatively, if you have the space on your HD, copy C:\ (or use a partition manager, and your right to use ythe upgrade is set up for you.