That systems pretty old and has poor hardware. If the memory is less than 1 GB (1024 MB) it will not be compatible. The standard memory was 128 MB is pretty pricy these days as its obsolete: [URL] Also likely it has an IDE hard drive which are no longer being manufactured and is the most likely component to fail. Taking into account the price of a Windows 7 license and the price of any possible spares and also the likelihood of components that old such as the hard drive failing, my advice would be to purchase a new system with Windows 8.1 such as a Dell Inspiron 3000 Desktop: [URL]~ck=mn#!dlpgid=everyday-desktops These are the lowest end Dell models that are currently on sale but will run leaps round the Dimension 3000. If a new system is too expensive you can look on ebay for a second hand OptiPlex 760 with Windows 7. Many businesses are upgrading to new hardware and selling off their older OptiPlex 760's which are great machines and far better than the Dimension 8100. The price of an OptiPlex 760 with Windows 7 included is actually not too far off a Windows 7 license. For instance: [URL] [URL]  
Can I upgrade my Dell Dimension 8100 from Windows XP to Windows 7? 
 Windows 7