Now if you are new to the term Botnet, let me give a brief description, its the network of many Bots that can range from few hundred to millions. A Bot is a short name for digital Robot, so its a machine capable of doing automated tasks once instructed by the Botmaster ( who controls the complete Botnet ). You can read the full detail here @ Wikipedia
To be secure over the internet you must follow the basic security guidelines like using a good Antivirus with a good Firewall. If you can afford its always suggested to use a nice Internet Security Suite. You should always use updated Softwares including your AV and Operating System as well. Do not download stuff from unknown or malicious sites either its Movies, Games or Softwares as I already told before no one is free to upload the same for free. Neither a student has time to do the same instead of compromising the study time nor an employed person or self owned businessman will upload the stuff for you.
If you keep on downloading the stuff from the internet for free and now worried if your PC is a Bot, you can check the same by using the free software RUBotted from Trend Micro. Once a Computer is a part of the Botnet, it is engaged in suspicious activity on the internet and for the same it sends unusual traffic. RUBotted monitors the same and if it finds any malware, cleans the same using Housecall.
Its in beta phase and as if now only supports 32 bit version of Windows. It uses WinPcap to capture network traffic, read more about the WinPcap functionality on the developer`s page itself here or @ Wikipedia. You can download the beta version of the software from the below link
Click Here To Download RUBotted Software
While installing the software, you will also see the installer for WinPcap from CACE Technologies ( Company owns the Software )
During the installation, dont forget to check the box that says load the driver at boot time
It will display that botnet malware found or not. If it finds the same at any given point, will show a popup to you. Even if it shows that no botnet malware is found and you doubt your computer performance for suspicious activity you can check our Malware Removal Guide.