There's a tool called Partition Magic, and a few free alternatives that allow you to resize your current partitions. You'll probably want to defrag your Winblows partition first, as it'll stick data where it pleases on the drive. Once you've done that, run the program, resize the partition to 160GB, and reboot into Gentoo. Linux tends to be less of an ******* when it comes to installing as a secondary OS, so, should be fine.