As mfinni has pointed out you can do limited server tasks on a standard PC, like share printers and share the hard drive. This is called a workgroup and is the default configuration for a windows PC.
I wouldn't recommend this setup for more than 5 or so users though. The reason being is that if the computer that is acting as the server is rebooted, then printing will stop and any open files will no longer be accessible and may get corrupted. For that reason it's important that files are copied locally, edited and then copied back if you're using that one machine as the file server. Do not open files directly off the server.
Running a full windows server inside a virtual pc session I could never recommend either. The performance will be marginal and will have the same issues as in the workgroup setup. i.e. that you can't reboot the machine without taking down the server. The other issue is that if you use domain control, the local PC will have issues accessing the domain controller until it's started. Fortunately, there is caching of the credentials but there might be problems there.