hmm boot from LAN, possible but running 2 pc's from one HDD is not a good thing at all. HDD's are very cheap these days. just build a new system
it really would be tricky to setup, you would need to load windows on the HDD twice for a start, configure to boot on lan in bois...... na don't even consider it. not to mention the extreme poor performace you will get from sharing a HDD.
MUCH easier to build a new pc than set that up.