Depending on how intensive the tests you need to run are, you can download a free version of the latest MS SQL from Microsoft for free. There is a limit on the maximum size of the database, and the number of users that can use the server at a time, but it is usually ideal for testing. As far as reusing the CD for a new install, MS has a lot of different licensing programs and you run the risk of activating the piracy check on a new install, especially for what sounds like an OEM license.
Microsoft SQL Express 2005 can be downloaded at
http://msdn.microsoft.com/vstudio/ex...r/default.aspx by clicking download in the upper right corner.