|The OS is on the old HDD, so if you have no disc, you have no way of reinstalling it. There are ways around that but we're not allowed to discuss software pirating ;-)|
Linux is a free OS though, would you consider giving that a shot? Linux Mint would be a good choice:
Another would be Mepis:
Both are very Windows-like. What you would have to do is download the .ISO file, then burn to a disc using a program such as ImgBurn:
Both are "Live CDs" which means you can just boot off the disc & try it out before you install it to the HDD.