Trojan scanners
Most virusscanners include trojan and malware detectors. Different companies have even stopped producing trojan scanners or stopped support of their current products. This will create some doubt about what to purchase, if you do want to buy a program, there's a lot of good free scanners out there. A all-in-one package is easy to handle but you run the risk of not finding every malware on your computer, just as no virusscanner will detect 100% of all viruses spread at the moment.
So what to do? Spread the risks and buy seperate programs or buy a bundle? The disadvantager of a bundle is, as I said above, no program will detect evrything. And if you happen to buy the bundle with the least updates or quality, you can get hit twice, both by viruses, trojans and malware. However, a bundle will take the least resources, norton not included in that statement ;-). Still I'd prefer a virusscanner/combined trojanscanner. Most people haven't even noticed most/all virusscanenrs also detect trojans, some evend etect malware. Guess we'll just have to wait for the perfect, well functioning, all-in-one suite but I doubt there ever will be one.
Remeber, a good trojan scanners scans the file when downloaded, opened, accessed etc. So a on-demand scanner is a must if you haven't got any trojan detecting virussoftware.
For info about what a trojan actually is, what it does and doesn't do go here