Your computer will open the compressed file (zip file) and you'll be able to double-click on the installation file named something like mbam-setup-1.50.1.1100.exe to install MalwareBytes onto your computer.
Follow the installation instructions to install and update MalwareBytes, the installation is very safe and simple, simply click Next and answer Yes or Run to all questions.
Once installed, MalwareBytes should run automatically. If not, launch it from your desktop by double-clicking the red MalwareBytes Anti-Malware icon.
On the Scanner tab in MalwareBytes, choose the Perform quick scan option and click Scan. This will scan your computer for most common viruses and malware in about 10 to 20 minutes. Once finished, you'll need to click Show Results and then Remove Selected.