Windows MX/PartitionMagic

Anyone out there who has experience with using Partition Magic in Windows MX? I wanted to create a new partition on my only harddisk of the brand new computer, and when I try to downsize the C: partition I get an Error 27 - Cannot Lock Drive. There is an explanation of this error message in the User Manual, but no solution! Anyone had that problem and found a solution? I absolutely hate having to put my data in the 'my documents' folder on C!

Many thanks for any hints...