Place a shortcut to the VM's VMC file in the host's Startup folder.

You can use the Task Scheduler to accomplish this. Basically you'll need to
create a command file on the host machine. To do this open notepad and type
the following into the open window:

"C:\Program Files\Microsoft Virtual PC\Virtual PC.exe" -pc "DOS
6.22" -singlepc -launch

The quotes are necessary due to the spaces in my path and naming convention.
Change the path and pc name to match your own path and VM name. Once you
have done this save the file to the local hard drive, I use a folder
directly off the root of the C: drive called Tasks to save these types of
files in. Name it something that will make sense to you later, like "Launch
VM DOS6_22.cmd". I'm assuming you already know that by giving it a .cmd
extension the file becomes a command file and can be run by double clicking
it. TEST the file by double clicking on it and verifying the VPC starts the
VM for you. If it doesn't edit the file and verify your path and name are
correct. If the file doesn't run, verify that the file extension is correct,
Microsoft did us no favors when they decided to hide the file extensions
from us, and notepad might have added a .txt to the end of the file for you.

Once you have your command file working properly, open your control panel
and select the Scheduled Tasks icon. Be patient while it populates the list
for you. Select the Browse button and then go to where you saved the file
above and select it. From the next window select the "When my computer
starts" option. You can also give the job a name at this point, but it will
usually use the file name and should already make sense to you. On the next
window you'll need to decide which credentials to use to start the job, it's
best to use some that don't require the password to change periodically, and
even better would be to create a local account or network account
specifically for scheduled tasks to use, and then restrict it to the
appropriate permissions on the box.

Finish the wizard and test your job by right clicking on the job and
selecting run. It should start. If you used a different account to run the
job under then you are currently logged in as the job will start, but you
won't be able to see it and you won't be able to launch Virtual PC in your
session since it's already running on the host machine. Only one instance of
virtual machine can run on a host machine at a time.