Go to your Start>Run and type the following exactly between and without the
Quotes ("") "REG DELETE HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System
/v DisableRegistryTools"
and/or
"REG DELETE HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System
/v DisableRegistryTools"

For standalone Windows XP systems, perform the steps below to remove the registry editing restrictions.
Method 1: Using the REG.EXE console tool

1. Click Start, Run and type(copy/paste) this command: REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableRegistryTools /t REG_DWORD /d 0 /f

OR use this script
Click here to download
OR
Click here to download (RS link)