Backslash of course – the slash key is used to separate windows domain name and user name when logging on. In this short tip the problem is simple – when trying to enter the backslash key you are getting something else for example # instead.
The particular problem has shown up several times after creating a new VM for a client and after rebooting it when it has just joined to the domain. You could always using the screen keyboard if using Windows 2008r2 or higher but this tip is even quicker. using the alt key and the code 092 (i.e pressing and holding the alt key and entering the code) you will get the \ backslash key at the insertion point.
For a more comprehensive list of the ALT codes – browse on over to http://www.alt-codes.net/