Where can I find what license key was used in my installed copy of Visual Studio 2008?

just run your computer and then type the word "regedit"you'll find the four basic HKLM but i thin k just use this :

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\9.0\Registration\2000.0x0000\PIDKEY on my PC. not sure but just try because some of this working.

If you are using Win7, then try HKLM\Software\Wow6432Node\Microsoft\VisualStudio\9.0\Registration\2000.0x0000\PIDKEY

A couple of options listed here:

Magical Jelly Keyfinder

Magical Jelly is the best product key finder for Windows. It’s free and portable, hence you can use it without installation or modifying the Windows registry. It can be used to extract the product keys of paid software like Microsoft Office, Microsoft Visual Studio, Adobe Photoshop etc.

and

WinKeyFinder

WinkeyFinder is another great program to find the CD keys of installed software as well as the operating system. You can use this program to

• Find product keys of Windows XP, Vista, Windows 2003, Windows 98 and Windows ME editions
• Find serial keys of Microsoft Office 2000, 2003 and Microsoft 2007.
• Change or copy the volume serial number.
• View .NET key.

You could try ProduKey from NirSoft. Its an app for listing Microsoft licence keys, however I don't think it'll work if your copy of Visual Studio was pre-pidded, i.e. it came from say an MSDN account.