Windows Virtual PC Quirks and Workarounds

If you’re moving from Microsoft Virtual PC 2007 to Windows Virtual PC on Windows 7, get ready for a few changes, plus some things that just don’t work quite right, especially with undo disks. After experiencing the pain of sorting it all out, I thought I’d write this post in the hopes that others can be spared the same grief.

Don’t get me wrong. Overall, I find Windows Virtual PC has some very nice enhancements over Microsoft Virtual PC 2007—particularly the new Integration Features, which supports guest machine access to host machine resources via a network shares (replacing Virtual Machine Additions that supported drag-and-drop between host and guest machines). But here are a few things that just might trip you up at first…

Shutdown options have been reorganized

I find the new arrangement annoying, despite claims that it’s now “less confusing” ( In any case, you won’t find the option to commit changes to the undo disk when shutting down the virtual machine. Instead, you can choose only to hibernate, shut down (properly, keep undo disk), turn off (pull the plug on the virtual machine, keep undo disk), or turn off and discard changes (pull the plug and delete undo disk), and to make that choice your default for the future:

To commit the undo disk changes, you need to revisit the Settings for the virtual machine, open the Undo Disk settings page, and finally click Apply Changes:

Pain in the butt, if you ask me, but such is life. According to Microsoft, this is “easier,” so who am I to argue?

Finding the Undo disk location

According to the docs, the .vud undo disk file is created and stored in the same folder as the virtual machine configuration file. That’s a bit confusing, because there are actually two virtual machine configuration files: the .vmcx file that is visible in x:\Users\YourName\Virtual Machines, and the .vmc file that is stored in the hidden folder x:\Users\YourName\AppData\Local\Microsoft\Windows Virtual PC\Virtual Machines, and the undo disk is stored in the latter (hidden) location. If you open the .vmcx file in Notepad, you’ll see that it refers to the .vmc file in the hidden folder:

For example, after starting virtual machine SQL08DEV with undo disks enabled, you won’t find the .vud (undo disk) file alongside the .vmcx file in x:\Users\YourName\Virtual Machines:

Before banging your head against the wall or jumping to the conclusion that Microsoft’s documentation is wrong (and I did both), just navigate to the real (hidden) directory and you’ll find it there alongside the .vmc file (and the .vmc.vpcbackup [backup] and .vsv [saved state] files):

Undo disks won’t merge if vmcx/vhd files are on separate drives

In Microsoft Virtual PC 2007, you gain superb performance by running your virtual hard disk (.vhd file) on an external hard drive (or even a flash drive), while the VM configuration (.vmc file) remains on the system disk. An undo disk automatically gets created in the same folder as the .vmc file (the system disk). The VM operates in this mode by exclusively reading from the external disk’s .vhd file, while simultaneously writing to the undo disk’s .vud file on the system disk—ipso facto, super-fast.

While this mode is still supported in Windows Virtual PC on Windows 7, there seems to be a bug that prevents you from eventually committing the undo disk changes back to the .vhd file on the external drive. No matter what—even after rebooting—Windows Virtual PC insists that the parent disk is inaccessible, and refuses to perform the merge. Of course if you don’t mind discarding the undo disk, this isn’t a problem (discarding the undo disk is a wonderful strategy for rolling back to a consistent state that sets the stage for a predictable and repeatable environment for presentations, development, or whatever). But when it comes time to merge, you’re hosed! Until they fix this, I’m forced to keep my .vhd files on my system disk (performance isn’t too bad though, on my screaming new Dell M6400 Intel Core Quad 12GB RAID 1 monster). Of course, if you’re not uncomfortable directly editing the XML in the vmc file, you may be able to shuffle the vhd back and forth between external and system disks. Hardly an elegant solution, and certainly not one that I’d recommend.

Integration Features won’t work if the virtual machine user account has no password

Integration Features in Windows Virtual PC has some nice advantages over the VM Additions in Microsoft Virtual PC 2007, but you will run into a problem enabling Integration Features if your virtual machine logs in with an account that has no password. That shouldn’t be a problem in most cases, since user accounts usually have assigned passwords, even on virtual machines. But some virtual machines, such as those used for demos, may have empty passwords. Unfortunately, the error message that Windows Virtual PC throws at you when you try to enable Integration Features on such virtual machines is anything but helpful. It would be nice to get a simple message telling you to just assign a password to your virtual machine user account. But instead, you simply get a failed authentication message with no indication that all will be fine if you just assign the missing password. You can try till you’re blue in the face (like I did), or just assign the password and move on.


Virtualization is awesome, and overall, Microsoft has done a good job of it with their Virtual PC products. Still, nothing in this life perfect—including software—so you need to watch out for these pitfalls when moving to Windows Virtual PC on Windows 7.

2 Responses to “Windows Virtual PC Quirks and Workarounds”

  1. Irina Says:

    Thank you for sharing! I’ve been banging my head against a wall for weeks on this stuff!!!

    Question on undo disks…
    Once I do ‘apply changes’ my ‘undo file’ doesn’t disappear/erase. Are we supposed to manually delete them after ‘applying changes’? There are several of them piling up and taking up my memory (which I’m short on…unlike you with you super nice monster! :).

    Thanks for sharing your experiences and saving others of us from the same plight!

    • Leonard Lobel Says:

      Glad you found it helpful. To be fair, I’ve since switched to Oracle VirtualBox. I find that it works better overall, plus supports 64-bit VMs on Windows 7.

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