I noticed that there is strange directory of C:\Users\<username>\AppData\Local\VirtualStore. Whenever I use cdimage to write files to an iso at C:\, it comes into that directory instead. Well, I can stand that...
However, recently I want to backup the settings of a certain program installed at C:\Program Files with 7-zip, and I happen to find that the files that are compressed are actually the old files at C:\Users\<username>\AppData\Local\VirtualStore\Program Files. I have to delete all the files under VirtualStore in order to backup the real files, and I haven't found out the side-effects of the deletion... (Hope nothing goes wrong!)
How can I prevent the system from writing to VirtualStore instead?

This is a feature of user account control (UAC) and from what I've read doesn't apply to Vista users. That folder will not be used if user account control is disabled.

The other way to disable it is to take ownership of the folder and set full control to the user you're currently on.

Thanks. Setting full control solves my problem. Maybe that's why the program didn't pop up a window to ask for admin privilege before writing. Well, I hope MS will ask before writing to such a secret place in future OSes.