 Do a system restore, pick a date before things got wonky.   All Programs>Accessories>System Tools>System Restore.  Thanks for the suggestion I've since found a solution to the problem, may prove useful if others suffer same issues:  [URL] 
I need to reinstall Teredo Tunneling Pseudo-Interface so that I can use Homegroup in Windows 7, as it requires IPv6. IPV6 is selected against Network adators etc How do I reinstall Teredo Tunneling Pseudo-Interface so it shows in netwok adapators in Device manager, please? I have my Dell Windows 7 Upgrade DVD but don't know how to retrieve the file from this. Possibly a Dell-Techie reading this maybe able to help?
Teredo Tunneling Pseudo-Interface Needs Reinstalling in WIndows 7