I can only have one IE version installed, but I need to make sure that webpages work on various IE versions, how can I do that without other computers?
Download the Internet Explorer Application Compatibility VPC Image that you require. These are time-limited virtual machines, but should suit your needs.
There's also IETester (http://www.my-debugbar.com/wiki/IETester/HomePage) which I found works pretty well.
The browsershots.org also may be useful. It shows how your web page will look like in different browsers. All IE versions are in the list.
IE collection is much more stable than Multiple IE, get it from http://finalbuilds.edskes.net/iecollection.htm
And if you need an application that can automatically take a screenshot using all these browsers, check http://www.browserseal.com
The most straightforward way is simply to install a virtual machine and then install several versions of windows, each with different versions of MSIE. I use VirtualBox for this purpose. It also then allows you to install several different browsers and versions at the same time (that is, it is a general purpose solution to such a problem).