Unfortunately, you can't backup the iPad without using iTunes. You also can't update to iOS 5.1.1 from iOS 4.x without iTunes. So, let's see...
It sounds as if you have a permissions issue with your computer and iTunes. To make a backup, iTunes makes a folder within your computer and sticks the backup in there. If you don't have your computer's permissions set up right, that may be preventing iTunes from creating that folder. Realize, of course, that this is my SWAG, here.
Do you have the most recent copy of iTunes? If not, get the most recent from Apple and, when you go to run the set-up, run it as Administrator. That might help your permissions.
That's all I can think of at the moment. Hopefully, someone else will come along and add more to this (if downloading iTunes didn't help). Sorry, but that's all I've got...
Marilyn