My operating system is WIndows XP sp3. The problem is that I can't see the hidden folders. The folder option tab is there but when I press it I can't see the hidden files and folders. It must be some kind of virus. I saw on the Internet about modifying the registry but it didn't work.
Can you suggest me any way to get the problem solved permanently without having to reinstall XP???
You made sure to enable the correct folder option, right?
tools-->folder options-->view-->click the circle to 'show hidden files and folders.' If you want all folders to be that way, click the 'apply to all folders' button. Make sure to hit apply, and click ok.
If that doesn't solve it, and you think you have malware, run through the malware guide in my signature.