How to remove a Nhatquanglan virus on the Guest account?

Question

Hi. I've tried the ComboFix+Win XP Recovery combination to rid my drive of Nhatquanglan virus and it cleaned my Admin account as well as my brother's. But for some reason, it didn't clean the Guest account. I can access the Folder Options and Task Manager all right, but I can't acess the command prompt and it infects USB drives.

Though I've disabled the Guest account for the mean time, knowing my PC's still infected with this stupid virus bothers me big time.

Help.