The registry is modified

My computer has been infected, I can manually modify the RUN item, but then automatically refresh to its original value, may I ask What is the reason.Note:Please do not tell me to download an antivirus software, I just want to know the operation principle of the virus.(I do not have to restart the registry and the computer).I have used a virus Wsyscheck the end of the process (to build a safe environment).

Because of the existence of the virus process, which is monitoring the startup items.When they find the relevant registry values are deleted or modified, it will immediately re-established.The solution is  anti-virus first, and then delete the System Startup Items.