This error, with the following error message, occurred right at the beginning of the installation Microsoft SQL Server 2005
 
"SQL Server Setup unexpectedly failed. For more information, review the Setup summary log file in %ProgramFiles%\Microsoft SQL Server\90\Setup Bootstrap\LOG\Summary.txt. "

Solutions:

1.) Is the Protected Storage service running on your PC?  (Reference: http://support.microsoft.com/default.aspx?scid=kb;en-us;246183)

a. Start the Protected Storage service by using either of the following methods:
• Use the Compmgmt.msc snap-in.
• Type the following command at a command prompt: net start "protected storage"
 
b. THIS WORKED : REBOOT Machine after making registry changes.

Check the registry and see if this key exists:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders\AppData

Both should be a REG_EXPAND_SZ and both should be set to %USERPROFILE%\Application Data

You might have to set the AppData key to %USERPROFILE%\Local Settings\Application Data

c.) MAY BE NECESSARY : Note If the Crypto subsystem has become corrupted, you may have to back up the following subkey, then delete it, and then restart your computer:
HKEY_USERS\.Default\Software\Microsoft\Cryptography\Providers\Type 001

 

Note If this registry key does not exist, proceed to the next steps:

2. Test to determine if the issue is resolved. If the issue is resolved, skip the remaining steps. If the issue is not resolved, go to step 3.
3. Start your computer in Safe mode, and then log on as administrator.
4. Double-click My Computer, and then click Folder Options on the Tools menu.
5. On the View tab, click to select Show hidden files and folders, and then click OK.
6. Delete all of the files in the "C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\S-1-5-18" folder.
7. Restart your computer, and then run the task.