It should be a basic partition and the empty space should be adjacent.
http://technet.microsoft.com/en-us/library/cc771473.aspx tells more.
If you can't get it working, your two options are:
1. Try if GPARTED does it?
2. Copy all the files out of that 70%-partition (to an external hard disk or so), delete it, create a new 100% one and copy the files back.