What is Hamlet's state of mind when the performance begins in Act 3 Scene 2 of Hamlet?

Such a loaded question!  For some reason, my mind spun
your question in a completely different way (so much so that I actually laughed when I
read it).  My first thought was, "People have been fighting about this since it was
written, . . . and they still don't know the answer!"  Namely, what
exactly is Hamlet's state of mind after he says
this:

As I
perchance hereafter shall think meet / To put an antic disposition on.
(1.5.171-172)

True, this is
said far before Act 3, Scene 2, but it is important because some critics argue that,
from that point on, Hamlet doesn't just act crazy, he actually
goes crazy.  This arguement is expecially significant because in
the scene previous to the one you mention, Hamlet says
this:

Go to,
I'll no more on't; it hath made me mad, I say we will have no moe marriage.
(3.1.148-150)

Mad in truth,
or mad in jest?

To further the inquiry, at the beginning of
Scene 2, Hamlet says some things to Ophelia that are quite unbecoming of a gentleman: 
things like "Lady, shall I lie in your lap?" while he shows signs of nervous tension
like you've never seen.  On the other side is the idea that Hamlet is obviously "acting"
a bit nuts here, and his nervous tension is the result of the knowledge of The
Murder of Gonzago plot:  a plot that will surely reveal the truth behind
Claudius' actions.

Therefore I must concur with the first
answerer, the reader/watcher must infer Hamlet's state of mind; however, we may never
know exactly how Shakespeare wanted us to feel about it because, luckily, this scene can
be acted in completely different ways.  Thus, the bard and playwright of the Elizabethan
Age keeps things interesting for us centuries and centuries
later!