Download and run the Windows 7 Upgrade Advisor to see if you computer is capable of running Windows 7:
http://www.microsoft.com/en-us/download/details.aspx?id=20
Assuming it is, you'll then need to get hold of a copy of Windows 7. Microsoft no longer sells it but there are plenty of online sellers offering it. Such sales are usually via downloads, rather than by supplying the discs though, and I most strongly looking for good reviews before deciding where to buy from (as there are a lot of dodgy sellers of discontinued Microsoft products). I'd advise seeking out the 'Professional' version as it offers 'Compatibility Mode' if you experience any problems with older software.
As long as the Windows 7 Upgrade Advisor reports that you can use the 'Upgrade' method (rather than only 'Custom Installation') everything on your Vista PC will be preserved.