How easy is it to replace a motherboard in a notebook?

[StoneAge]

I have an HP zd7000 pavillion notebook. Almost on cue (just before the one year warranty expired) the motherboard went bad. HP repaired it and sent it back.

A short time later (after the warranty expired) I started having some problems and took it in to the local computer shop- they said the motherboard was bad. I asked them if there was a possibility that there was some problem causing the motherboard to go bad, as I had just had it replaced. They said the most likely scenario was that when I sent it in for warranty service, the motherboard was replaced with a used board that was on its way out.

I figure my options are:
1) have it replaced by a shop- $450
2) get a new computer and hope I don't have any problems
3) replace the motherboard myself- how easy is this to do? I generally am ok at fixing things. For example I do most of the repairs on my vehicles myself- and seeing as this computer cost more than any vehicle I have purchased ($1800)they have definitely needed some repairs. Or am I just asking for an expensive lesson? I assume I can find a board online for a lot less than the $450 the shop would charge to replace it.