Well, on this particular computer I do not recommend to run a third party firewall. Windows firewall is pretty decent and takes up less resources than third party firewalls.
For your antivirus I recommend
Panda Cloud Antivirus - it is the lightest of the free antivirus. Install it without the toolbars.
A second choice of antivirus would be
Avira.
Besides an antivirus you should install an on-demand scanner.
MBAM is the recommended choice. Use it to periodically scan your system.
WinPatrol alerts you when changes are made in vital system areas. Very light program, especially good on light systems not running a third party firewall.
Finally: browse safely. Google Chrome is the safest browser imo. Mozilla Firefox with the NoScript addon is also very safe, but Noscript has a couple of major disadvantages (it blocks legit content and if you decide to "temporary allow" stuff on bad sites, you open the door for infections).