Or..instead of doing a fresh install of windows and having to manually put everything back on.. get a Ghost bootdisk, and transfer from the 80GB to the 160GB. I've done this a couple times, and it's worked perfectly. It transfers everything, including the MBR. Then you can just format the 80GB one, and use it as like a data drive.
Edit: before copying, make sure your IDE is set to Slave jumper, and you'll have to boot off of a Ghost bootdisk. I'd reccommend trying to find a Ghost 2003 edition, because it's the easiest to do. Just boot off of the disc you make, and do Local > Disk to Disk > and select the source drive (make sure you select the right one..the only way you'll be able to tell is by looking at the size of the HDD), then select the destination drive. For me... it took me about an hour to copy 50GB of data from an 80GB IDE drive to a 320GB SATA drive.