+1 for avg from grisoft for everyday sitting in your system tray.
spybot from safer-networking and malwarebytes for your now and then scan.
if you have a usb modem rather than a router with a built in firewall i would then use the free version of zone alarm firewall.
all the above are free.
id also suggest using adblock plus and noscript add-ons in firefox rather than internet explorer,
get a gmail account to avoid the majority of spam mail, dont click on 'birthday cards' and other special offers you may get in your inbox. dont open attachments from people you dont know.
follow the above should keep it cleanish.