Any HDD should be properly prepared BEFORE an OS install as to how you want sizes, partitions etc.
This is very true, but in this case it's a simple oversight and the OP just needs to allocate space and maybe toss in a partition.
All the third party utilities claim to have non-destructive ways to resize/rename/move partitions etc blah blah blah....
In my experience none of these methods are worth it period end.
Well, I have pushed, pulled, bumped, divided and dissected hard drives with Partition Commander with no disasters yet.
That is why they can sell it.
The key is to stay within reason and not try to make a partition too small.
To the original poster, I was thinking Partition Magic (Ghost's stable-mate) and typing Ghost. (I just re-read)
Disk Management is fine for what you are trying to do.
Regards,
JF