I am assuming that you have an upgrade DVD. If so the answer is yes, if you follow this tutorial. Clean Install with a Upgrade Windows 7 Version
I would make an image of what you have. Make sure all important documents are backed up; including mail, etc. In addition use Windows Easy Transfer.
This is a surefire way of insuring that you will not lose anything during the upgrade.
The windows easy transfer, is really cool, everything but programs will be transferred, including desktop and favorites.
One other hint, when asked for your number during the install, do not give it. In addition uncheck the box for automatically activating. You will have 30 days to activate after the install. In this way if there is any question about the key, you can speak to a rep on the phone and he will activate you. Its easy and painless.