Hard to say exactly. Take a look in C:\Users[I]Your User Account[/I]\AppData\Roaming and see if you can find some accumulated files in the folders within that one.
Replace the words [I]Your User Account[/I] with your actual user account name.
This of course assumes that your 50gb drive is your C: drive.
Once you have some room, you might download CCCleaner and use it to make some space.