Though I know you do not want to, I'd also recommend a clean installation of Windows 7 (especially since your current system is not booting). Doing a clean install will allow you to start fresh.
You say you do not want to install a "new" copy of windows. You will not need to purchase a new copy of Windows 7. You can use the same copy you are using now, but you may have to activate it with a phone call to Microsoft. You can learn more about activation here, http://windows.microsoft.com/en-US/windows-vista/Activating-Windows-frequently-asked-questions
Hope this helps!
Microsoft Windows Outreach