I have mentioned in the past that Any Video Converter ( a program I quite like) asks to install extra software during the installation process. Nothing nasty, but it’s annoying to get an extra toolbar you didn’t want just because you didn’t pay attention.
Other programs can be sneakier. So if you find an odd browser toolbar or a strange new icon on your desktop it might be worth downloading and running Should I remove it www.shouldIremoveit.com
If you have teenage children I would definitely give this a spin.
It won’t pick malware, so make sure your your security software is up to date. If you are a Windows user get the free Microsoft Security Essentials.