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MS-DOS is no longer officially supported in Virtual PC 2007 but it does still work. Problem is, the DOS guest additions disk image is no longer present. This can be obtained from Virtual PC 2004 however.
To pull the DOS guest additions out of VirtualPC 2004, run msiexec /a "Microsoft Virtual PC 2004 MSDN.msi" first and extract the files to somewhere. Look inside the 'Program Files\Microsoft Virtual PC\Virtual Machine Additions\' directory in where ever you extracted the files to. The desired floppy disk image file is 'DOS Virtual Machine Additions.vfd'. Attach it to the Virtual PC 2007 VM and run A:DOSADD.BAT.
The DOS guest additions actually are still present in VPC 2007 but only on the CD image which is not so useful. Additionally, the install batch file is missing so you're left to install them on your own. As such, its just easier to pull the disk from VPC 2004.