To change drive letters (useful if you have two drives and have partitioned the boot drive, but the secondary drive shows up as "D")

Go to Start > Control Panel > Administrative Tools > Computer Management, Disk Management, then right-click the partition whose name you want to change (click in the white area just below the word "Volume") and select "change drive letter and paths."
From here you can add, remove or change drive letters and paths to the partition.