It doesnt make a difference if the drive is called C: or E:
Anyway,
Rearranging the drive letters is simple.
Go to control panel (classic view)> Administrative tools > Computer Management> Storage> Disk Management(local)> right click on the drive letter you wish to change and select "Change Drive Letter"
Screen-shot:
Hope that helps.
PS: If its bothering you because E: is bigger than your C: then try resizing the partitions using: GParted -- Live CD/USB/PXE/HD
@shaunak: You can't change the drive letter which contains the OS. You can able to change the other drives, If you do so, it will throw a warning message.. Check for the screen shot here: Drive Letter Change on Flickr - Photo Sharing!