AVG Free for anti-virus. Make sure you choose the free version, no reason to pay for it.
Comodo Personal Firewall for firewall. Again, stay with the free version.
You also need to think about backup. Now is a great time to implement a good backup strategy that will protect you from a hardware or software failure in the future.
I'd recommend getting an external USB 2.0 drive enclosure, with fan and at least a 250GB hard drive (IDE). The you can buy Acronis True Image or Norton Ghost or again go the free route as I do and use DriveImage XML.
If you go the DIXML route then you will want to also get and build Ultimate Boot CD For Windows (UBCD4WIN),. It is a bootable CD that you create on your computer using your own copy of Windows XP. One of it's included tools is DIXML so in the event of a hard drive failure you just replace the drive, boot the UBCD4WIN CD and restore from your external USB drive.
Keep the backup current. I keep two full backups and replace the oldest. I do this usually once every two months unless I've made some significant changes to my system in which case I do it immediately.