Crashed OS, Windows Key

First you need to load the registry on the crashed system, this is best acheived by using hirens or plugging the drive with the crashed OS in to another machine.

Quote :

Go to Start => Run and type in regedt32
Select the hive HKEY_LOCAL_MACHINE
Go to File and click on Load Hive
Navigate to the Registry Hive you want to edit (e.g. D:\Windows
\System32\Config\Software).
You will be asked for a key name to load the Registry Hive into. Give
it a name such as "Import".
The imported Hive will show up in the Registry tree and you can edit
it like any normal hive.
When you're finished editing, go to File and click on Unload Hive to
save the edited hive.

Then follow this tutorial below on how to find the registry in the loaded hive


http://teamtutorials.com/windows-tutorials/retrieving-xp-cd-key-from-the-registry#.T0bNePWfXZY