Like WAMP allows one to run a Unix-like webserver on an existing Windows PC, is there any way to setup a Windows Server installation on an existing Windows PC?
I'm looking at re-using one PC as a server, while allowing the user to work normally with his Windows apps.
You might find it simplest to simply run Virtual PC on the computer and virtualize the Server instance. Of course, you could also use VMWare Server, but I was thinking Virtual PC for simplicity's sake. Either way, make sure you've got enough spare RAM. 512MB at the least, preferably 1GB.
OK, since you used wamp as an example (apache, mysql, php) I assume you're talking about web apps? You can use Microsoft's free Web Platform Installer to do something similar. Use it to get IIS, SQL Server (express) and just about everything you need to host web applications on a windows machine.
It will install on XP, Vista, 7 as well as the Server OS's.
Can you describe, in detail, what you want the "server" to be doing? Is it going to be serving files or printers? You can do that with non-Server Windows just fine, it's only to a limited number of concurrent users though. And performance can be affected -if the local user is doing 3D rendering, the network users will be affected, and if network users decide to host their PST files on the share, the local user will be affected.
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.