You can't be sure if you dont read the source code or disassemble the software, so you basically have to trust the vendor. For additional security its recommended to scan your PC with scanners like AdAware ( http://www.lavasoft.com/software/adaware/ ). \nThis still wont give 100% security, because this program can only find most Spyware, but not all.
How can I tell if my software includes spyware or adware?