You can use VMWare or Microsoft's Hyper-V or any other virtualization software. Microsoft's Lab Manager gives you even more and uses Hyper-V and there are other similar tools as well.
One thing that I would point out though is that you have an assumption that you need to include OS in your matrix of testing web sites. I would assert that a spot check would be fine for any Windows based OS and you don't need to include different OS's in your normal rotation. In my 13 years testing websites the number of issues found in different OS's is miniscule and in my opinion including those in a test matrix is a huge waste of time and resources better spent elsewhere.
One exception to that is if you are using ActiveX controls, Flash, Silverlight, etc. which could function differently on different OS's.
Another exception is non-PC platforms (Mac/Linux) where obviously you will have different browser versions which could also function differently. I have definitely seen some differences in functionality and rendering between Safari/Firefox on Mac VS PC. For your question, you will want to include those in a test matrix. Keep in mind from an automation standpoint though that while your automation will find functional issues, you still need to do a manual spot check in those browsers for any rendering issues anyways, so again trying to put a lot of effort into automating that process may not be that cost effective.