Virtual PC 2004 (and 2007) do not have native PXE booters, which is a bit of a nuisance.
Although there is no sign of PXE support being implemented, Roudy Bob has come up with the PXE-Enabled boot disk for Virtual PC. This is a virtual floppy of a PXE boot disk that will allow your Virtual PC to connect to PXE servers.
Which is nice.