If you have a router or hardware firewall, I would say the Windows firewall would be adequate.
For anti-viruses, I recommend Kaspersky or NOD32. Both are excellent anti-viruses and neither take up a lot of resources (NOD32 is more efficient than Kaspersky, however).
Right now, Futureshop has Kaspersky Anti-Virus 7 on for $30 and TDDirect.ca has NOD32 for $27.50 + shipping.
Buy NOD32 Antivirus Ver 3.0 1 Year Subscription OEM English Edition Antivirus Antiroot Kit Future Shop: Software: Utilities: Kaspersky Antivirus Version 7.0