I currently have Internet Explorer 11 installed, but I need Version 8 or 7 for some onlinework.
Do I have to uninstall Internet Explorer 11 prior to installing 8 or 7?
You can't install old versions of Internet Explorer on modern versions of Windows.
Microsoft publishes free virtual machine images which allow you to launch a virtual computer running whichever version of IE you want: http://www.microsoft.com/en-gb/download/details.aspx?id=11575
It's a bit of a pain, but the only way to truely test how a site works on <=IE7 as the emulation in modern versions of IE is not 100% reliable.
In my experience, Internet Explorer is difficult to uninstall but is relatively easy to roll back to previous versions. Depending on what version of (presumably) Windows you're running (I'm using Windows 7 Professional x64):
From here you can roll back to an earlier version of Internet Explorer. Each time you remove an IE update, IE will revert to the next-earliest version (so for you starting with IE 11, you will first go back to IE 10, then 9, ect).
There's an article on microsoft.com which describes 5 of the more popular ways to test websites with different versions of internet explorer: http://www.microsoft.com/en-gb/developers/articles/5-ways-to-test-your-website-in-old-versions-of-ie
Basically, those methods are:
There are also 3rd party browsers that you can download, that can emulate the rendering of all different IE versions. I can't say that those all work perfectly, IMO you're best of with the real deal (i.e. using one of the above methods).
F12 should open up the Development Console. Under Emulation you should see "User agent string". From here you can select the IE version to run under.
Note: This did not work that great in IE10. I have not tested this in IE11. The problem was with how IE8 handled things like positioning in CSS and how the emulated IE8 handles them. There were minor differences at best, though.
Assuming that the pages were designed to work with the rendering or the Javascript of previous versions of IE, then IETester is a good choice. From the site:
IETester is a free (both for personal and professional usage) WebBrowser that allows you to have the rendering and javascript engines of IE10, IE9, IE8, IE7 IE 6 and IE5.5 on Windows 8 desktop, Windows 7, Vista and XP, as well as the installed IE in the same process.
It isn't recommended to downgrade IE versions, mostly because by downgrading you are losing the security improvements over the older versions.
See this website on how to run multiple versions of IE
Utilu IE Collection contains multiple stand-alone versions of Internet Explorer, which can be used at the same time. It supports Windows 98 to Windows 8.1 – but currently supports only Internet Explorer 1.0 to Internet Explorer 8.0.
More information here