ProcNetMonitor is a portable tool which helps you to view your PC's open network connections.
Just launch the program and you'll see the usual list of items: open browser windows, your email client maybe, anything else which happens to be running and connecting across your local network or the internet.
Need to know more? Click one of the connections and you'll see details such as the port being used, and the remote host address (the website name, perhaps, for your browser connections).
You can also search for connections using a particular port. Type 80 in the Search box, say, then click "Port Search" to highlight your browser processes (and perhaps any others which might be trying to hide on the HTTP port).
If you think you spot malware, right-click a process and select Scan Online to check VirusTotal or ProcessLibrary for more information. And if it turns out to be malicious, the right-click option also has a Kill option which will terminate the offending process.