"I really don't think you'd want to do this. It makes it a lot easier to break things on the computer since everything can be overwritten.
However, if you need to do this for some reason, you can right click the C drive, and go to properties then the security tab, then give Alex write permissions. Now click advanced and check off the box to replace permissions on child objects to effect files recursively, it will take a while to complete. It is the same or close in XP I believe:"