Spyware, viruses, & security : Is someone else using my computer?

Is someone else using my computer?

I have a Dell desktop with XP. I run Avast, spybot and BlueCoat. I do not have passwords on my computer, but that will change... My problem is that I think someone else is using my desktop computer....today (I am at work, the computer is at home), some of my web mail got downloaded into my Outlook (I think) between early this morn and lunch... (it's not some one breaking into my house, but let's put that aside for now)

I know Bluecoat has a log of internet usage, but if this person is booting up my computer to look at my e-mails, they will not (necessarly) use the internet.

Is there a wndows log of bootups that I can look at to find out the last time someone was on my computer? or is there a log for Outlook (2003) that would state last time it was opened?

Any idea of a good tracking program I can put on ?

Thanks in advance...

- Mijo