Oh happy days - SQL Express 2008 has failed to install with the following error:
"Rule 'Consistency validation for SQL Server registry keys' failed". The SQL Server registry keys from a prior installation cannot be modified. To continue, see SQL Server Setup documentation about how to fix registry keys.
The most likely cause is registry permissions, DO NOT try some users advice of uninstalling all copies of SQL Server, at least not until you have checked the registry permissions.
You will find the installation log in Program Files\Microsoft SQL Server\100\Setup Bootstrap\LOG folder. Open the log and check for any errors in the steps relating to registry key checks/updates. If the errors relate to security or access permissions this is your problem. Open registry editor, locate the key indicated in the install log, right-click and select Permissions, change the permissions at the very least local administrators have full access. After doing this for each affected key try the install again.