Microsoft registers valid licensees via a unique pin-code (known as a 'Product Key') which comes with your software purchase. The product key (see the example below, where 'X' denotes how the Key Code would appear) is usually a 25-digit code, comprised of letters and numbers; it comes with both Windows installation CDs, and Windows downloads.
If you have the CD version of your Windows check the packaging; does it carry a certificate of authenticity? (see the image below for details)
Check to see whether you Windows software has a holograph on the CD-Rom which came with it. The holograph should be a coating attached to the CD, and not an adhesive sticker or label.
Check the outer casing on the installation CD-ROM; does it have an outer ring (see below) which should read 'Windows Genuine' when held up to the light (or moved from left to right to reflect the light)?
Check the inner holographic band on the CD-ROM; this often (but not always) has a series of triangular pointers. This area may also show various hallmarks such as the Windows logo, or a statement that the CD-ROM is genuine.
- The problem may simply be that you have accidentally triggered a problem during installation so try un-installing and re-installing the software or re-inputting the product code.
- If this fails, you will need to contact the retailer you purchased your software from, or contact Microsoft directly to ascertain the validity of your product key.