Virtual box guest operating system will not start

A good way to troubleshoot this is as follows:

Back up your virtual machine.
Uninstall Virtual Box and any VB bits if there are any.
Restart the machine.
Install Virtual Box (newest edition) and patch.

Now see if your machine starts.

Separately, I keep backups of all my machines in case something like this happens.

Mr Hurst:
  Thank you for responding on a Saturday.  My knowledge of Virtual Box is small.  
Please tell me more about what files to backup to make sure I have a complete backup of Virtual Box .
When you say “restart the machine”  do you mean Virtual Box or the host operating system?

After I install a new version of Virtual Box , what must I do to connect the new version of Virtual Box to the Virtual Box files I backed up?

Mr. Vermaak:
  No, unfortunately, no snapshots are available.

I upgraded Virtual Box to the latest version (5.1.18) and the problem still occurs.  It seems the problem is in the guest operating system, not in Virtual Box .  Now 2 computers are affected.  Both of these have Virtual Box version 4 installed. They have worked well for over 2 years.   There is one computer with version 3 of Virtual Box and it's been working well for over 5 years.  

I have contacted Virtual Box support.  They responded to several questions but now I get no response.  So I figure I should create a new entry in Virtual Box and install Windows 2000 .  

Don't know what else to do.

You may need to reinstall the virtual operating system. There is not much else you can do.

Did you do a BIOS updated on host?

Shaun:
  No, I have not updated the bios on the host computer.  It's a Dell 7010.  There are 4pc's in the office and 2 of them are running well.  They all have the same bios.  

John:
  On the pc which Windows 2000 will not start:  As I mentioned, I upgraded Virtual Box v4.2.18 to v5.2.18.  In the Virtual Box manager, I created a new virtual machine since the original would not run.  I attemped to install Windows 2000.  The process gets as far as the Windows 2000 splash screen where it will start installing Windows 2000 for the 1st time and the installation process stops there.  

So, now, I'm really clueless what to do.  My guess is to completely uninstall Virtual Box and install the newest version and then try installing Windows 2000 again.  This is really strange.  I have a pc running version 3 of Virtual Box and it has been running Windows 2000 for 5 years.

I wonder if there is some incompatibility with Windows 2000 and version 4.2.18 of Virtual Box. I don't know how I would construct a search query to learn if there is such an incompatibly .

What ideas do you have to solve this problem?

What other free virtual machine software is there which would run Windows 2000?

I think there is a free (or very inexpensive) version of VMware Player from VMware.com. I have VMware Workstation Pro (not free but best of breed) and I have a working Windows 2000 machine here.

As I mentioned, ..... I created a new virtual machine since the original would not run.  <-- My apologies, I neglected to go back and look.

I do not have these issue with VMware Workstation as I noted above.

PROBLEM SOLVED!!

I uninstalled all the antivirus programs and Windows 2000 and Virtual Box run just fine.
My client had installed 2 anti virus programs on his pc.
I had to remove both.
I installed Security Essentails anti virus.
Everything works now.

Thanks to all who responded.  I pay for this site so I appreciate your prompt responses.

Glad we could help. Will remember this issue for future.

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Thank you for the update.