Can't virtualize my bootcamp win 7 installation because of changed size or something?

In order to solve a bug of Parallels that can't virtualize my bootcamp win 7 installation i read here that i should just decrease a tiny bit the bootcamp partition with iPartition so it re-syncs the MBRs or something.
When i went ahead to do that i get:

"iPartition was unable to get write access to the device Hitachi (disk0). This may mean the device is write protected, that you do not have permission to access it, or that it is in use by something else."


It's definitely not being used by anything else, i can browse it so i guess i have access. It's NTFS though.