Yes, you realistically have two options here
1> Create a separate user account. By doing this, you will be sharing the same windows installation with them, and like Ghastly said... while it will help, it still won't protect you should they pick up a virus, or some kind of malware that messes up the Windows installation.
2> Create a dual-boot system. This basically entails two separate windows installations. Yes, you can use two separate hard drives... or you can just use the one by partitioning it (you can create a 50-100GB partition for your needs and windows will see it as another hard drive, even though it isn't).
Then you install Windows onto the other partition and when you reboot the computer a menu will appear just before windows loads asking you which one to boot from. If you don't touch anything, it will default to the first one, so you can set this menu to appear for only 1 second, and if someone didn't know it was there, there is a good chance they'd never even notice. Seems like a decent way to keep the family out of your gaming side of things! Make sure you put a password on your gaming account setup so that if they do happen to find it, they won't be able to mess with it. Also, you can set up your partition/drive to not show up on their Windows.
I would advise creating a third partition on the hard drive as well, to keep back up images. After you get everything installed and set up, along with updates, etc... you'll want to make a backup image of each hard drive (or each partition if you go that route) and save it in the third partition you made. I would also advise burning these backup images to DVD's, an external hard drive, or at least a flash drive... just to be safe. Storage space is so cheap nowadays, there really is no excuse to not do this.
Yes, to go with option #2 you'll need another copy of Windows.
Amazon.com now has the FAMILY PACK with THREE Windows 7 upgrade disks for $125! Yes, they are upgrades but believe me... don't let that stop you. You can still do a full install with it (at least I could last year with the family pack I bought).