Recently I had to add compatibility for IE6, IE7 & IE8 to a site I was working on. Only problem was that I can only have one version of IE installed on my machine.

STEP 1: Getting things running

1. Download & install Virtual PC
2. Download the IE VPC images you need from Internet Explorer Application Compatibility VPC Image.
   
   Note: You don't need to download the version that matches your operating system since each VPC image contains a full version of the OS.
3. Once you've downloaded your images, click on the .exe to extract them to any folder on your computer
4. Launch Virtual PC (Default Location: C:\Program Files\Microsoft Virtual PC\Virtual PC.exe)
5. From Virtual PC Console, click New...
6. Option: Select Create a virtual machine option and click Next >
7. Name: Name your New Virtual Machine, ex./IE6 or IE6 XP SP3, and click Next > Note that it does ask you for 'Name and location' but you just need to specify the name here, later on it'll ask for the location
8. OS: Choose 'Window XP' as the Operating System (or whatever OS matches the images you downloaded) and click Next >
9. 'Use the recommended RAM' Next >
10. 'An existing virtual hard disk' Next >
11. Location: Browse to the location on your computer you extracted your image earlier. Next >
12. Done: Finish to close the wizard and create a virtual machine with the settings you specified

Repeat the wizard for as many images you need to install.

You can start your new VPC image anytime on your computer by selecting it from the Virtual PC Console and clicking on ‘Start’.

STEP 2: Getting localhost to work

With the installation above you're ready to start using your installed images. Problem is that each image is their own installation of Windows. So for testing locally you can't just type 'http://localhost/my-site' and see it as you do normally.

1. Get the IP address you need to access: From the CMD prompt type 'ipconfig', use the IPv4 number near the top (you many need to scroll up)
   
   Note: You can access the CMD prompt by either clicking start and entering 'cmd' into the search pane... or by going to C:\Windows\system32\cmd.exe
2. If you haven't already launched Virtual PC and select one of your images to start.
3. Type the IP address you found via the command prompt into the address bar, you should get a Forbidden (403) error. This is because Apache isn't configured to accept requests from the IP being used by VPC.
4. Open your Apache access log (may take a moment)... scroll to the bottom for the last access... should be something like
   192.168.0.10 - - [25/Aug/2009:10:02:50 -0400] "GET / HTTP/1.1" 403 202
5. Open your Apache httpd.conf, Search for 'Allow from'
6. Add a new line underneath with the IP address from the Apache log Allow from 192.168.0.10
7. Restart Apache

Now the IP address of your VPC image is allowed to access your localhost... hit refresh on the browser in VPC.