The answer can be found if you read the magazine 'Aeris.' You have to read the whiteboard in the lab twice to find the message at the bottom saying that Yoshida borrowed the magazine.

After that, you have to go back up to the sleeping quarters on B1 and check Yoshida's bunk. You'll find the magazine, which counts as a key item. If you read through the entire thing, it will say that the virus changes the password by taking all of the letters in it and putting them in alphabetical order, then putting the number of times that letter appears. It also says that the virus then puts the old password on the computer screen as a background.

If you look at the computer's login, you see Melissamaya, so the password would be A3EILM2S2Y.