How to shrink Windows 7 boot partition with unmovable files

Problem

I have just bought an HP laptop with Windows 7 (64 bit). It has a 500 GB HDD with three partitions: a small hidden system partition, a 12 GiB HP recovery partition, and a 450 GiB C: boot partition. I would like to split this large C: partition into two partitions, leaving only 100 GiB for the system, and giving the rest to a new data partition. Although the Windows built-in Disk Management utility has an option to shrink the bootable partition, it only allows me to shrink it roughly by half, even though only 20 GiB on the partition is used. As far as I understand, system unmovable files lie in the middle of the partition, preventing Disk Management utility to do what I want. And since new HP laptops don't come with OS installation disks (they only allow you to create recovery disks yourself), I can't just repartition HDD and then reinstall OS. So, is there a way to shrink the C: bootable partition and preserve Windows 7 working? P.S.: I have tried to use the third-party GParted utilit…