Ahmed needs to reinstall Windows, but his hard drive no longer has the necessary recovery partition.
If your computer came from a major manufacturer, the hard drive has some important tools that you won't find on drive C:. And you're only going to miss them when you need to reinstall Windows.
Companies like Dell, Gateway, and Lenovo ship their computers with a special partition that contains an image backup of drive C:--as it was when the PC left the factory. When you tell your computer to reinstall Windows, it restores everything from this backup, returning your PC to its factory condition.
That's why you should never format or wipe your entire hard drive, but only the C: partition. And if you upgrade to a better hard drive, you need to clone all of the partitions to the new drive--not just the ones with your programs and data.
If you no longer have that partition, contact the PC's manufacturer and explain the situation. In most cases, they'll be willing to sell you the recovery files on DVDs for a reasonable price.
If they're not, there are other options. Borrow a retail copy of your version of Windows (such as Windows 7, Business Edition) and--after backing everything up--install from that. But when the program asks for your Product ID number, don't give them the one on the disc jacket. Type in the number on the plate attached to the back or bottom of your computer. This isn't supposed to work, but I've heard stories of it working, anyway. If that fails, call the Microsoft 800 number on the screen, and explain your situation to the first human being you get. This has worked for me.
Whether you buy the discs from the manufacturer or borrow them from a friend, read Reinstall Windows Without Losing Your Data before you start the installation.
Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at email@example.com, or post them to a community of helpful folks on the PCW Answer Line forum. Follow Lincoln on Twitter, or subscribe to the Answer Line newsletter, e-mailed weekly.