Hello,
Welcome to Microsoft answers forums!
Do you have Windows vista installation disk?
I would suggest you to try this out first:
Open the Task Manager (Ctrl + Alt + Delete), then the tab processes, select and end explorer.exe. Then click a menu in the toolbar, select and run as a write explorer.exe, then accept.
This process will make the Windows desktop restarts.
If not,
Check what happens when you try to boot in safe mode.
To start your computer in safe mode, you can refer the below link:
http://windowshelp.microsoft.com/Windows/en-US/help/323ef48f-7b93-4079-a48a-5c58eec904a11033.mspx
If you are able to boot to safe mode, try the following changing registry settings:
You have to set the default value for the windows shell setting:
In the Windows Registry (open 'regedit')
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\winlogon\
Set 'Shell' variable = 'Explorer.exe'
REGISTRY EDIT DISCLAIMER:
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows:
http://support.microsoft.com/kb/322756
Hope this information is useful.
Regards,
Savan – Microsoft Support.