Good idea of log63 but it does not work for me.
I used the Junction program to create a link to a LARGE disk I have and that I use to backup all the iPad information to.
If a use dir for the amount of space in the "C:\Users\Ashe\AppData\Roaming\Apple Computer" directory I get 15,089,598,464 bytes free -- not enough to install iOS 5.
But if I ask for the stoage space under the junctioned "MobileSync" folder under the above structure I get
1,508,029,829,120 bytes free -- plenty to install anything BUT the Apple upgrade program will not follow the link as does iTunes when backing up -- frustrating.
I hope Apple fixes this shortcoming soon -- I'd like to try iOS 5.