the Non-Existence of Virtual PC Express
According to older news Windows Vista Enterprise and Windows Vista Ultimate would both have a feature called Virtual PC Express. In fact both versions of Windows Vista do not ship with this feature. Clearly the cause of this is the new virtualization path Microsoft has chosen nearly nine months ago. Virtual PC Express, which was rumored to only allow one Virtual Machine (VM), would be a useless feature since Virtual PC 2004, Virtual Server 2005 and Virtual PC 2007 can be downloaded for free.
I've seen a couple of persons in the Microsoft Newsgroups asking where they can find this feature. After receiving the above information the main reaction is disbelieve, disappointment, but fortunately not a reaction stating Microsoft tricked anyone into buying a more expensive version of Windows Vista... Here's why I think it's not only a logical choice, but also a good choice not to add Virtual PC technology to Windows Vista Enterprise and Windows Vista Ultimate:
Windows Vista Ultimate
If you'd buy Windows Vista Ultimate the promise was you'd get Virtual PC Express. Even if you wanted to use it to run a Virtual Machine, you couldn't use it to run a Windows (Vista) one. This would conflict with the End User License Agreement (EULA), which allows only one running instance (Not literarily from the EULA, but it is what it boils down to...). You'd legally have to buy an additional copy of Windows. Since this looks like something that can be easily overlooked it's better Microsoft didn't add Virtual PC technology to Windows Vista Ultimate.
Windows Vista Enterprise
If you have a Windows Software Assurance contract you're entitled to Windows Vista Enterprise. You could have used Virtual PC Express to run a Virtual Machine. You could have used it to run only one Virtual Machine though, which is the main difference between Virtual PC Express and Virtual PC 2004 and Virtual PC 2007. According to the End User License Agreement (EULA) however you're entitled to run four, regardless of the virtualization program used.
"But ... you could have used it for temporary issues..."
According to the Microsoft website Virtual PC Express' purpose was to facilitate non-Windows Vista compatible programs. That would be nice if it had additional benefits, like Group Policies (I guess Darren would've loved to see that!) or Active Directory Schema Extensions to store information on Virtual Machines in Active Directory (Tomasz, I haven't forgotten your excellent piece on the Schema Extensions for Wireless Networks and BitLocker Drive Encryption!), perhaps even an extremely locked down version of Windows XP already installed, purely designed to run applications and nothing else (Now that would be secure, wouldn't it Paul?) or have an add-on Active Directory Application Mode (ADAM) database to authenticate to for Active Directory integrated applications without actual integration or authentication to your physical Active Directory...
Since Virtual PC Express doesn't have any of these features it's as useful (or useless) as Virtual PC 2007.
Virtual PC 2007
Three weeks after the launch of Microsoft Windows Vista, Microsoft Office 2007 and Exchange Server 2007 on January 30, 2007 Microsoft made Virtual PC 2007 available. This version of Virtual PC 2007 is optimized for use with Windows Vista and allows you to do everything you ever wanted to do with Virtual PC Express.
"But ... Virtual PC Express would run every version of Windows..."
Yes, that's what Microsoft promised. The Virtual PC 2007 virtual machine additions however won't work on Windows 95. (you can use the Virtual PC 2004 virtual machine additions though) I think I can say beyond any doubt that Virtual PC Express ship with the same support for Windows 95. (although initial rumors state Windows 9x support.)
You won't find Virtual PC Express in Windows Vista Enterprise or Windows Vista Ultimate. If virtualization is your goal just use any recent version of Windows and any recent version of virtualization software. Don't make virtualization a buying point for Windows Vista Enterprise or Windows Vista Ultimate.
I think it's a shame Microsoft put a lot of effort in announcing Virtual PC Express, but virtually no effort in letting it dissapear gracefully...
Virtual PC for free!
Microsoft Software License Terms Windows Vista Ultimate
Microsoft Unveils Windows Vista Product Lineup
Expanded Benefits in SA Deliver Support Across All Phases of Customers’ Software Lifecycle
Virtual PC 2007's Home on Microsoft.com
Get Ready for Windows Vista Enterprise Edition
Windows Vista Enterprise on Ryan Glover's virtualization blog
Virtual PC round-up on James O'Neill's blog
The Windows Vista SKU plan on Don Roessler's Blog