From a purely legal standpoint, what you want to do would qualify as trespassing. Depending on state, or country, there could be more specific laws with stiffer penalties.
From an armchair therapist standpoint, think about what you're doing for a second. Assume he went through a bunch of your private stuff, how would you feel about that? Probably a bit hurt, betrayed, angry... And you'd have every right to feel that way, and so would he if you broke into his email account.
If you have a solid relationship, this will all pass. At the very least, your psycho stalker girlfriend act isn't going to help matters any. You need to keep trying to get him to open up to you, and tell you what's going on. If you can't do that, you don't really have much of a relationship to begin with.
this has got to do with computers but just please bare with me and read till the end. kay so i love my boyfriend soo much we have known each other all out lives and now im broken hearted because he has left me because my friend has been saying things to him he wont tell me what shes said and when i try talking to her she dont want to know and i just really cant cope is there anyway i could get into his e-mail just for a bit to see what it is because i need to know and i think my minds going crazy i love him soo much more than anything we played together as children grew up together why would i want this to fade away over something shes said to him over msn please help i need to get into it to talk to her to see whats what