I`ve had some experience of this too. I suggest you install the trial version and then remove it from Add/Remove immediately after rebooting the computer. The trial version is on a hidden partition which is why it isn`t in Add/Remove at the moment. You may not need the link given by PaulB2005. Avast is a good choice.
If you are concerned about resource usage, it may be worth paying a modest sum for NOD32. Probably the best AV going and lower resource use that anything else, Avast included (which is not quite as low-resource as one might think).