This kind of warning error comes , when your system registry got corrupted or some Virus ,Trogen effect, but if you know its solution to remove Virus and you want to enter in Registry, but you can't, as registry writing is also blocked
The Virus will not only block your registry, but also you can'nt able to open your msconfig,taskmanager.
For temporary solution to get the access to your registry, you can download a XP Emergeny Utility Tool
you can download it freely, i have given the below link from there too you can download it
After downloading it, its a executable file, just run it, now you can able to get access to your registry, msconfig and taskmanager.