Cheaper than a mac with the same affect install Linux.
Otherwise try these should be enough
Good antivirus theres to many to mention but most of the free ones like avast and avg are actually beter than norton and the rest.
spybot search and destroy
http://www.safer-networking.org/en/download/
use this to imune and scan for spyware
Spyware blaster
http://www.javacoolsoftware.com/
use this to immune you more from spyware
ad aware
http://www.lavasoftusa.com/software/adaware/
use this for scanning for spyware
apart from that you can use a trojan scanner like Tauscan which is what i use or the cleaner which is pretty good to
keep all of the above updated scan once a week at least if your parinoid
of course you can use mozilla even Linux or a mac but that only protects you and you might miss something like an IE exploit or a dialler