First, let me say I am by no means an expert.
That said, I beleive, there are different levels of security and someone can always get in sometime, heck even the pentagon has been hacked before.
Regardless, there are spyware programs out there, the most common adaware by lavasoft and spybot. Also MS allows a beta spyware download from their site. I use a program called Counter Spy by Sunbelt Software, and it has found things, including key loggers, that none of the others I just mentioned found on my system. I have used all of the above and Counter Spy is in my opinion the best of those I have used, (which surprised me over MS's program).
Don't know if this helps. If you have that sensative of information I would be running a spyware program every hour, lol. You might ask someone about firewalls and other programs that create secure tunnels, such as Zone Alarm, but I am not sure how much they can protect against spyware. I know virtually nothing about them. Hope this was helpful in some way.
My biggest fear is for some type of adware or spyware to be able to pick up my keystrokes when I enter into my bank account, etc.
I have everything program wise that says "secure" but if someone can pick up my keystrokes, wouldn't I be dead in the water no matter what "protection" I have????