There are 2 possibilities:
First backup your files.
Then download the live cd from gparted, burn it to cd, and boot from the cd
Partition you drive according to your needs,
First install windows then linux.
Or use gparted to decrease the windows partition, then increase the linux partition.
Before shrinking the windowspartition, you first need to defrag the partition.
Please note this can cause dataloss, so backup first.