I just purchased a 320GB Western Digital SATA hard drive from newegg.com and it was about 135$ me thinks (more expensive because of SATA).
You can get good deals on hard drives, and if you can install a dvd burner this is just as easy. You secure the drive with mounting rails (if your case has them, my Antec P180 does) or screws, plug in the SATA or IDE cable and attach the SATA or IDE power cable. Just be carefull that if you connect if via SATA (small rounded cable) that you dont use the IDE power (4 pin molex). If you connect it via IDE (40 pin flat grey ribbon) that you use the IDE power (4 pin molex). The sata power cable looks similar to the SATA motherboard connector.
Removing your movies should have free'd up a bunch of hard disk space, how much total and free hard drive space do you have. And you have just one hard drive correct???