How can i change the save directory for an iphone firmware update 2.1?

Yea my itunes wants me to download and install something 2.1. My C disk is nearly full so i want to download the update to my D disk. how can i change where it will download/save to?How can i change the save directory for an iphone firmware update 2.1?
You can simply use the following link to download the firmware manually and save it to a location of your choice, but when you want to update your iphone to 2.1 click SHIFT + Restore and browse to where you saved the file.



2.1 for iPhone 3G:

http://appldnld.apple.com.edgesuite.net/content.info.apple.com/iPhone/061-5198.20080909.K3294/iPhone1,2_2.1_5F136_Restore.ipsw



2.1 for iPhone 1st generation:

http://appldnld.apple.com.edgesuite.net/content.info.apple.com/iPhone/061-5202.20080909.gkbEj/iPhone1,1_2.1_5F136_Restore.ipsw