I have a protagonist that needs to invade the antagonist's computer. On that computer, in Quicken, is the antagonist's financial records. Protagonist cannot enter the antagonist's home. Protagonist does have the ability to get help from people who know what they're doing. Protagonist does have a granddaughter who can enter antagonist's house and use computer (antagonist is her dad and protagonist's ex-son-in-law). However, granddaughter cannot knowingly help protagonist.
I'm thinking of accessing computer via the internet. Possible?
Yes, it happens all the time.
http://en.wikipedia.org/wiki/Zombie_computer
One need only send an email with an .exe attachment and convince the receiver to 'click on' it. If the mail program is Microsoft Outlook or Outlook Express (or an equally badly configured email program set to automatically runs attachments when you read an email(!!!)), it isn't even neccesary to get the recipient to click on anything, just open the email. And likewise with websurfing, no doubt there are Java and script programs that will save a file somewhere so that it will get executed the next time the machine boots. I know with Windows 2000/XP/Vista it's more complicated than that, but there are still lots of ways to fool the OS into running a program the user didn't manually install.
What I bolded is the critical part. I know this can be rather easily done technically (writing the program would not be easy, but others have already done it), there are huge numbers of zombie computers out there (you could be reading this on one!), and the people who control the computer have full control over it and can read any file on it (totally unknown to the person who owns and uses it - the 'control' programs run in the background, are well-written so the users don't notice any slowdown, and they only send spam in short spurts so the ISP doesn't notice a large amount of outgoing data).
But I don't know quite where or how to get such programs, and I'd rather not go poking around the cyberneighborhoods where they might be found. Getting access to such a thing might cost hundreds of dollars, maybe thousands (remember this is highly illegal, the people who know how to do this make a living at it, so they've got a lot to lose - they'll go to great lengths to protect their identity), and there's never any assurance that "the people who know what they're doing" aren't the police or FBI, and likewise if they're real
they don't know if
you aren't the police or FBI, though they may well check up on your credit history, who you work for, etc., but even then they know all that can be faked by a really good police agency.
So it seems to me the complications in getting this done are much more social than technical.
I saw "Sneakers" as well, but the discussion also reminds me of the start of Terminator II, where the kid has this thing to get money out of ATM's. He gets a stolen ATM card, connects his gizmo to it, and it has an LED numeric display that goes through all 10,000 possible PIN numbers (0000 to 9999) in about five seconds. When it hits the right PIN the ATM spits out the requested amount of cash. Well, as you might expect, things don't quite work that way. The ATM sends the entered PIN (after encrypting it, supposedly) to a main computer. If it matches the saved PIN for the account it takes the funds out of the account and tells the ATM to dispens it. If not, it tells the ATM it's the wrong number. This back-and-forth takes at least a second, and after maybe five bad attempts at the PIN, the computer locks the account so it won't let you get money out even if you DO know the right PIN number.
ETA: a couple more "fun" URL's of fact, not fiction:
http://en.wikipedia.org/wiki/Storm_virus
http://www.physorg.com/news114790175.html