If the BIOS is showing only 160 GB, there's something else wrong other than bad partitioning. Does your BIOS show the hard drive model, or just the detected size?
If the BIOS is detecting only 160 GB, you might want to try running Seagate's SeaTools diagnostics and see what it finds. There is a version available on Seagate's website that can be burned to a blank CD and booted off of, so Windows dows not need to work.
I agree with cklaubur, even after partitioning a drive, it should ID itself to the BIOS as its maximum capacity. Did you have 2 or several partitions on the drive prior to it's unusual 160GB size, and how much dat is supposed to be on the drive.
Also be careful to back up any data on the HDD before manipulating the drive, so that if the Seatools writes to the drive you won't lose any data.
If you can, also go to Disk Management, and upload a screenshot of the Disk Management dialog box, and post it so that we can see exactly how the OS is recognizing your drive.