Not only does 3DS Max take very long to render the image, it's making even simple tasks like browsing extremely sluggish while it renders...
Why does this happen, and is there any way to make my computer useable when I am rendering?
Computers are generally only good at one thing at a time. Your 3DS Render probably has a normal or high process priority, so the OS process scheduler is prioritizing your render (which is both CPU and GPU intensive).
You can change the process' priority to run "Below Normal" or "Low", and also restrict the number of cores that it can execute on. That way, your browser will be scheduled higher in the process execution schedule, and be guaranteed to run when it needs.
Alternatively, you could just run your web browser at a higher process priority then the render, so the computer prioritizes that instead.
If you run Windows, go to the task manager (CTRL + SHIFT + ESC), and go to the Processes tab. To set the priority of a process, right click and go to Set Priority; to change the number of cores it can run, click Set Affinity instead. To launch a process with a set priority/affinity, see my answer to this question (ignore the HyperThreading stuff).
On Linux, you can use the