I do not have enough free space on the C:\ drive of my Windows computer to run the updater for iOS 5 in iTunes I am trying to update to iOS 5, but iTunes wants 10 GB in the C: drive. (I don't have that much free space in C, but I have it in another partition.)

I have already reinstalled iTunes in my secondary partition (300 GB free) and moved the Preferences > Advanced iTunes Media to my secondary.

But still when I try to update to iOS 5, iTunes tells me there is not enough free space (it still wants to use the C: drive).

Are there any other settings I should check?
I think that iTunes is trying to use your gobal Windows temporary folder. By default, this directory is in your user Local Settings directory, located on the same drive as Windows. This is a system-wide setting that applications are supposed to follow.

You can change the path to this temporary folder by changing the value of the TEMP environment variable. You'll have to restart iTunes after changing the variable to have it take effect.