|Then you are not seeking to "change your drive letter",|
only change the destination folder for your downloads.
What browser or download manager do you use?
The exact alteration to settings will depend on that but the interface should be fairly intuitive.
eg. In Firefox look in Tools>Options for destination folder for saved downloads.
In IE7 initiating a download brings up a dialog box asking where to save so you can alter the destination. It remembers the last setting used and defaults to that unless you change it. If you are using a different browser or download manager then look at the settings options.
As to having a drive "chock FULL", if it is full to more than 85% of it's total capacity you risk a crash and I suggest you consider moving some files to free up space that will be needed by windows for defrag & swap file purposes.
Goin' Fishin' (Some day)