What ever security setup you use always put some imaging software like paragon or macrium reflect so that even if you get infected somehow you can revert back to an earlier image. Also since you are in xp i might suggest to add a firewall. Try some of the firewall like online armor, comodo or zonealarm and only use that firewall which you find easy to use because if you are not able to configure a firewall properly it wont be protecting you effectively also always run your browser sandboxed with sandboxie for extra protection.
edit: just noticed you said your cousin uses torrent please tell him to stop because its the first place to get infected.
My Computer
At a glance
windows 7 ultimate 32 bitIntel core 2 duo T6400 @ 2.00 GHz2048 MB DDR 2 399.0 MHz
I would recommend using Avast Internet Security with sensitivity for all the protection modules set on high. Also, I would force all his web browsers and P2P program into Avast sandbox. Get Hitman Pro for 2nd opinion and Malwarebytes for weekly scan.
My Computer
At a glance
Windows 7 Home Premium x64 SP1Intel Core i7 2720QM @ 2.20GHz8.00 GB Dual-Channel DDR3 @ 665MHz