I am suggesting that torrent clients are capable of bypassing OS X's protections, and that anything downloaded from such sites should be considered unsafe. This is especially relevant for someone who would characterize himself as layman with computers in general.
If you are not absolutely certain of what you are installing on a computer, its intended purpose, what you expect it to do, how to use it, and how to get rid of it, don't install it. OS X's built-in Gatekeeper is intended to limit the availability of programs to recognized developers, and its built-in Xprotect normally checks for known malware every day. Both of these features are circumvented by downloading torrents. It's your Mac, and nothing can prevent you from doing that if you so choose, but you must accept responsibility for whatever havoc can occur as a result.
You have a number of Internet add-ons that may be problematic, as well as some third party system modifications such as NTFS-3G, MacFUSE, and Growl that may be outdated, and could be responsible for your complaint. To uninstall system modifications, you have to consult the developer's documentation. To uninstall Internet add-ons in Firefox, use Tools > Add-ons and uninstall what you don't need. Having said that, the largest unknown is the torrent client and what it may have installed. It is difficult not to throw up my hands and conclude anything is possible with an unknown of such magnitude, which is the reason for my recommendation to erase your system and reinstall OS X. It may be overkill, but given this unknown factor it is the only way to guarantee a system that can be used and maintained in the way it was designed.
4 GB is not an abundant amount of memory but I have systems that run perfectly well with that much, without frequent appearance of the "wait cursor" as you describe. More memory may help, but it will only mask the underlying problems that must be addressed.