Please beware of downloading free software. Although not all free-ware is spyware, a good portion of it is. Therefore, if you are downloading a free pdf to jpg converter please make sure you get it from a reputable download site like cnet.com (download.cnet.com) or if need be pay for it. Otherwise you will be downloading spyware, or an annoying toolbar that will insert itself into your browser or something worse. The tradeoff between saving a few quid and the time spent extracting the spyware or virus from your computer is not worth it.
To download free software you can try cnet.download.com who advertise virus-free, spyware-free downloads of free or free-to-try software. I have not had any problems with their software.