Reset lost BIOS password – Virtual Server 2005

An answer to a question that came up on the internal Microsoft discussion group for Virtual Server 2005 amused me. The question was what could you do if you lost the BIOS password for a virtual machine.

In a physical machine, you can often move a jumper on the motherboard or remove the battery to reset the CMOS. Obviously a virtual machine doesn't have anything physical like this, but the answer posted up was to "remove the virtual battery". You can't, although I did look through the .vmc (Virtual Machine Configuration) file for a battery setting 🙂

The actual answer is simply to open the .vmc file in a text editor, and scan down to the <bios> section which will be similar to below:

        <serial_number type="string">1234-5678-4797-4379-2023-2540-66</serial_number>
     <bios_guid type="string">{5242D6D9-20F8-7442-B2E6-DFCFFC45749A}</bios_guid>
     <bios_serial_number type="string">1234-5678-4797-4379-2023-2540-66</bios_serial_number>
         <asset_tag type="string">1234-5678-4797-4379-2023-2540-66</asset_tag>
         <serial_number type="string">1234-5678-4797-4379-2023-2540-66</serial_number>
     <time_bytes type="bytes">55000100100005270105</time_bytes>
     <cmos type="bytes">00004000F02....truncated...000000</cmos>

If you remove the highlighted line above starting "<cmos" and restart your Virtual Machine, the password will be gone (as will any other customisations made to your BIOS). Note that this file is locked while the Virtual Machine is running, and this should also work under Microsoft Virtual PC 2004 as the .VMC file is largely compatible between the two products.

Comments (8)

  1. Eric Balk says:

    I am having some other problems with Virtual Server 2005 and find this article interesting for the fact that there is a discussion group for Virtual Server 2005.

    Where can i find that?

  2. John Howard [MSFT] says:

    Eric – I was referring to an internal to Microsoft discussion group. There is an external newsgroup publicly available through (if your ISP doesn’t have a news server) by the name of microsoft.public.virtualserver.

    This is also available through the web at

    You can also try me 🙂

  3. grat says:

    use Bochs!

  4. jelly says:

    hi. i just have a vhd image file. how do i access the configuration file, .vmc.

  5. John Howard [MSFT] says:

    A guest cannot run without a .vmc file – the .vhd file is just the hard disk. You can create a new guest and add your hard disk to it, in which case there isn’t an BIOS password to be reset.

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