If you are the only person who ever used this computer, then its possible that someone might have, however most hackers dont hack into a persons system to check their e-mail so this is really strange.
Its a greater probability that someone walked over to your computer running to check their own e-mail vs a hacker doing this. I'd delete cookies from browser and stored passwords and all and give it a couple days. I assume your system doesnt automatically logon and so someone cant turn your computer on and use it without you around? If it automatically logs on to Windows, I'd change it to require a password.
