That's why the options are so great. There is a solution for every work habit, need and budget.
The diff between Emulation and Virtualiztion is significant.
Virtual PC is emulation, it is an environment that runs an OS on non naticve hardware so all the API, hardware calls have to be "translated" from one "language" to another.
Parallels and VM Ware are true Virtual spaces. An OS running in memoery space on it's native hardware. All the calls are passed directly to the hardware, not translated.
VM's are a developers tool. They are allow you to try different hardware environments without the expense of the physical hardware.
Good quick read.
http://blog.1530technologies.com/2006/08/virtual_machine.html