As far as actual tips, keep in mind that a software layer AV product can't protect you from exploits that happen at the lower-layer operating system level.
Use Sandboxie, an older version of Comodo firewall (not sure if the newer version will work) and Immunet is what I use on the Netbook with XP SP3. I'm not sure if Bitdefender free will work on XP, but if it does go for that.
You will need to replace Internet Explorer 8 (which is 6 years old now and the last version of IE to support XP) with an alternative browser. Firefox still supports Windows XP SP2 and Opera also still works with Windows XP. You can use Google Chrome on XP but Google are only supporting it on XP for another month or so, so Chrome is probably out of the window.
If you are using Microsoft Office on your computer make sure you are using Office 2007 or 2010 if you plan to continue XP for some time yet. 2010 was the last version to support XP and will continued to be supported until 2020 providing you all of the Office 2010 updates installed (and 2007 is supported until 2017). If you don't have or can't afford Office 2007 or 2010, remove the older version of Office you are using and install the free LibreOffice or OpenOffice instead.
Asides from that, so long as you are smart and don't go on dodgy websites or anything like that and make sure that you have every security update for XP installed, you should be OK but do remember XP is now obsolete and you really need to think about upgrading in order to keep yourself as secure as possible online and to ensure that you can continue to enjoy using your computer. Whilst these programs still support XP now, they may not in a years' time and then you will be stuck.
How much longer are you intending to use Windows XP for?