I am at this precise moment a client to format a PC with Windows XP that was in need of a fresh-install after several months of intensive (not to be a Windows ...).
After making all the list of currently installed software and backups of personal data, I am faced with a problem: the label sticker on the PC box with the Product Key (also known as Serial Number) Windows is unreadable, which I impossible to re-install the operating system (at least in a 100% legal).
After a simple Google search found ProduKey I solved the problem in seconds.
This program can retrieve the serial numbers of various products, including:
Microsoft Windows 98/ME, 2000, NT, XP, Vista, Server 2003 and some versions of Windows 7;
Microsoft Office 2000 to 2007;
Microsoft SQL Server 2000 and 2005;
Microsoft Exchange Server 2000 and 2003;
For ethical and legal use of this software must obviously be carried out only on systems on which the user has authorization to operate.