[internet-explorer] how to install multiple versions of IE on the same system?

To answer your question: no, it's not possible to have multiple versions of IE (if that is what you meant) installed in a 'normal' way (i.e. not a hack, a sandbox or a VM etc). It's perfectly ok to have multiple browsers of different types installed on the same machine, such as IE8, Firefox 3 and Chrome all at once.

SandboxIE should allow you to install multiple versions of IE side-by-side (as well as other software), and this is less hassle than going down the virtual machine route.

However, from a QA point of view I'd strongly recommend installing different versions on different machines as the best option from a testing point of view. This will give you the most realistic testing environment. If you don't have the hardware for that, then virtual machines are the next best option as mentioned in some of the other answers.