This behaviour is observed with many viruses/Malware/Spyware which take the help of the registry and launch themselves during startup or during the initiation of a particular program. However, if the PC is handled by a pro, he can easily know the name of the virus, search the reistry for it and delete the items associated with it. Hence the virus disables the registry so that the user cannot hav access to it. Here is a way to get it back.

• gpedit.msc is used again in editing the registry. Go to Run --> type gpedit.msc
• Select the Administrative Templates in the User configuration, collapse the System option
• Look for Prevent access to registry editing tools option on the right and double-click it.
• Select Disable in the Radio Buttons provided and click ok.

Now the policy of preventing access to registry editing tools is disabled and thus the regedit can be used again in the run.

If that doesn't work out, try the registry cleaner here.

Pc Tools Registry Mechanic 2011 - 1 User/3 Pc