Rebooting a syatem is easy, just use the shutdown command:
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/shutdown.mspx?mfr=true.
PXE booting is a capability of the NIC though and has nothing to do with Windows so is not scriptable or controllable from Windows itself.
You do not have to PXE bot a system to use OSD though, simply advertising a task sequence to an existing system with the ConfigMgr client on it will kick off the OSD process on that system.
Jason | http://blog.configmgrftw.com | Twitter @JasonSandys