There are a few ways you can do it. I had a large hard drive so I choose to partition the hard drive and install win8 on the partition and now I can use either. Windows 8 comes with a boot screen so when it starts it ask you which OS you want to use.
Check out this read and it will tell you about a clean install or an upgrade. There are a lot of very experience people on here that may have a specific solution for you.
Check this link.
