Well, if the PC has a regular desktop OS, you can't install SQL Server unless you're installing the Developers version or the Personal edition (which was only sold with the Enterprise license).
Otherwise, SQL Server MUST be installed on a Server OS, regardless of whether the machine has desktop hardware specs or Server hardware specs.
Now, the ramifications of installing a Server OS on a desktop hardware spec machine are significant. Usually bad if the server is trying to handle any kind of load. But that's a topic for another forum. @=)
Catadmin - MCDBA, MCSA
"If a person is Microsoft Certified, does that mean that Microsoft pays the bills for the funny white jackets that tie in the back???