One of the things about playing with Software Restriction Policies is that you can go too far and end up not being able to do anything to rectify the situation. Active Directory Group Policy applied through a link to an OU is easy enough to fix - simply remove the GPO link and reboot the client. However, if you are experimenting with Local Policy, you can come unstuck. To resolve this: Boot into safe mode and logon as the local administrator. Start up the registry editor and remove all values beneath the HKLMSoftwarePoliciesMicrosoftSaferCodeIdentifiers. On a reboot, everything should be back to normal.