You cannot get a new one. You need the one from the COA sticker on your machine. If you don't have that you can either upgrade to Windows 7 (or 8) or install a version of Linux. Frankly, I think your best bet is the latter. New versions of Linux such as LinuxMint are extremely user friendly requiring only the most basic of understanding of Linux, and nothing really technical. You can learn what you need to know from a basic Linux tutorial within 30 minutes. Linux will perform better on the hardware you have and the vast majority of software is free. Most new distributions of Linux have a package manager included which will provide you lists of ten's of thousands of free applications covering just about every task imaginable and the application manager will install them for you.