This question has come up numerous times and is always worth revisiting.
Question: I purchased a copy of Windows 7 on eBay back in May. I was not able to install it until last week. The activation failed and said the code was not genuine (although it is on the box); the original buyer is no longer on eBay, and I have no way of contacting him/her. So is there any work-around, or do I have to bite the bullet and buy another version?
— Jerry
Answer: I see this a lot, Jerry. It happens more often than not when someone buys a newer version of Windows from somewhere other than an authorized dealer. Every version of Windows has a product key. Microsoft tracks the registration of those keys, and doesn't let a key be activated more often than the original owner paid for.
I'm not saying that whoever sold you that copy of Windows 7 was purposely doing something illegal. I can think of a dozen scenarios in which that person could have sold the copy of Windows without knowing it had been activated. Or without knowing it had a limited life. Your best bet at this point is to call Microsoft directly, tell them your story, and ask for an activation. No promises as to what's going to happen, but it can't hurt to try. Unfortunately, I can't just give you a phone number; the number you're given depends on a series of things. But here's how to find it for Windows 7:
Click Start. Right-click Computer. Select Properties. At the bottom of this window, click Activate Windows now. The Activation Wizard is displayed, select Use the automated phone system to activate. Use the drop-down menu to select the location nearest to you and then click Next. The phone number of a Microsoft Product Activation Center will be displayed. Call the number and follow their instructions.