I came across some news about the Twitter application being susceptible to web worms. This reminded me about the vulnerabilities that are introduced by installing third party software.
Installing any application on your computer can, possibly, open your computer up for attack or abuse. This is especially true in cases where the software is intended to communicate with other computers. In the past, music sharing software, and other social networking software has been vulnerable to abuse. If there is a communication link between your computer and another site, misuse can occur if the software you are running has any security holes, or is just poorly written.
Such abuse can take the form of outside access to your computer. In other words, someone exploiting a weakness in these packages, could possibly copy files, to get information from your computer without you being aware. Software can be installed on your computer without you being aware, in a similar manner. This could take the form of a virus, spyware, or even a keylogger. Keyloggers track all your keystrokes and sends them to someone else. I do not think that you really want the password to your on-line banking site sent to someone else.
So, be very careful what software you install, and, as always, keep the software up-to-date. Make sure that you have anti-virus, and anti-spyware, software installed and kept up to date.