There are several virtualization products that you might consider to use for your projects. Within the Microsoft family, Windows Virtual PC is the latest version hosted within the Windows 7 OS. This product is free and can be used to deliver Windows XP mode to enable legacy applications which sometimes do not run reliably in the Win7 environment. Virtual PC 2007 was the older version of this product used primarily in Windows XP desktops. Both of these products only support 32-bit versions of the guest OS and have limitations on non-MS operating systems. For those of you running 64-bit Windows Server 2008 and 2008R2 there is the Hyper-V server role which adds considerable power, security, performance, and management advantages.
Another open-source solution that is quite popular is called Virtual Box and more info on supported OS configurations can be found here.
This product can be hosted on different operation systems as well. For download information, go to: https://www.virtualbox.org/.