Assigning Drive Letters

I have a 120 gb HD which is all assigned to C drive i am trying to partition it with partition magic 8, but i seem unable to assign a drive letter before the CD Rom which is at the moment D Drive it is only allowing me to assign from F: onwards.

I have partitioned harddrives before but this is the first time i have come across this problem as usually the cd rom drive is moved along to the next letter.

any help you could offer would be appreciated.