A lot of malware, viruses and spyware disable the use of the Registry Editor to stop you from removing the offending software. If this happens, you can re-enable use of the Registry Editor by click Start and then Run and typing in:
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableRegistryTools /t REG_DWORD /d 0 /f
then click OK. Then you will be able to click Start and Run and type in
and click OK. You may need to do this quickly, as some malware detects the changes and disables it again.