It won't disappear after 7 years. Like someone said, the debt will change hands before 7 years with them passes, and therefore makes the debt appear to be new. I speak from experience. Things don't magically disappear after 7 years.
Now, what you need to do is contact the original utility company and ask to settle it with them. If it's with a collection agency, you need to fire them via Cease & Desist letter and that will kick your debt back to the utility company, OR another collection agency. Then you keep doing the same thing until it bounces back to the original creditor. You are only liable to pay whoever the debt was originally with.
You mentioned you are looking to buy a house. I highly recommend CTX Mortgage. Like most mortgage companies, they will run your credit and tell you what you need to pay to qualify. We did our loan with them completely by fax and we didn't even meet any of our loan people until the day we closed on our brand spankin'new house!!