HomeGroups in Windows 7 are awesome. Its an easy way to share content on your computer with other users on your network. There is a rather large issue with it though. Its not an issue with HomeGroups specifically but more with how modern games get around needing Administrative Privileges to run updates. Games like World of Warcraft install on Vista and 7 to C:\Users\Public\Games\. The problem is that by default when you join a HomeGroup this directory gets shared out with full read/write access to everyone in your HomeGroup. To prevent a malicious user from destroying your WoW install, you just need to change your NTFS permissions on that folder.
Start by opening Computer and browsing to C:\Users\Public. Right-click on the Games folder and go to Properties. In the Games Properties window click on the Security tab and then Advanced.
Our goal is going to be to remove the Everyone entry from the permissions list which currently has Full Control. Click on Change Permissions. Before you can remove Everyone from the list, you'll need to turn off inheritable permissions (currently Everyone can access C:\Users\Public\Games because they can also access C:\Users\Public) by unchecking the Include inheritable permissions box. It will ask you if you want to Add the current permissions to the new list you'll be making or remove them and start with a clean list, click Add.
Now scroll to the Everyone entry in the list and click Remove. To be safe, make sure that Administrators is in the list with Full control and/or your user account. If it isn't click add type in either Administrators or the user name you use to log into windows and in the window that follows click Full control and OK.
That's it! Go ahead and OK out of all the windows that are still left open. Now only administrators and people you added to the list will be able to access that folder. When someone accesses it from a HomeGroup they are using the HomeGroupUser$ account which is not an administrator and was being caught in the Everyone permissions before. With that gone your WoW install is now safe from being randomly deleted by bored network neighbors! If these directions weren't clear enough, or there's anyone questions, speak up in the comments.