By using Open office and Firefox, you are already into open source software - but it is still mixed in with Windows.
Can I recommend you request a Ubuntu 10.04 CD from
CDs and DVDs | Ubuntu If you install it, I also recommend that you ditch Firefox in favour of the Chrome browser. You will also get Open Office as well.
I have it installed with no problems and it boots up in 47 secs with a 9.7 sec shutdown! it does all that Windows can do and is free!
You have nothing to lose but Windows!!
Also meant to say, one does not need any anti-spyware, etc programmes as linux is 'immune' from these problems.