(Brian Reiter from thoughtful computing has described this setup in this StackOverflow thread. The credit for the idea is entirely his, I have just extended it with some step by step descriptions and added some links and screenhots.)
If you are forced to still support Internet Explorer 6, you can setup following combination on your machine to make the development for it less painful. A common problem when developing on Windows 7 is that you can’t install IE6 on your machine. (Not that you want that anyway). You will probably end up working locally with IE8 and FF, and test your IE6 compatibility on a separate machine. This can get quite annoying, because you will have to maintain two different development environments where you might not have all needed tools available etc. If you have Windows 7, you can help yourself by installing IE6 in a Windows 7 XP Mode, which is basically just a Windows XP running in a virtual machine.
 Windows XP Mode installation
After you have installed and configured the XP mode (remember the security settings like Windows Update and antivirus software) you should add the shortcut to the IE6 in the virtual machine to the “all users” start menu. This shortcut will be replicated to your windows 7 XP mode start menu, and you will be able to seamlessly start your IE 6 as a normal window on your Windows 7 desktop.
 Configure IE6 for the Windows 7 installation
If you configure your XP – Mode to use Shared Networking (NAT), you can now use IE6 to browse the sites on the internet. (add proxy settings to IE6 if necessary).
The next problem you will confront now is that you can’t connect to the webdev server which is running on your local machine. This is because web development server is crippled to allow only local connections for security reasons. In order to trick webdev in believing that the requests are coming from local machine itself you can use a light weight proxy like privoxy on your host (windows 7) machine and configure the IE6 running in the virtual host.
The first step is to make the host machine (running windows 7) reachable from the virtual machine (running XP). In order to do that install the loopback adapter and configure it to use an IP which is routable from the virtual machine.
 How to install loopback adapter in Windows 7
After installation, assign a static IP which is routable from the virtual machine (in example 192.168.1.66)
The next step is to configure privoxy to listen on that IP address (using some not used port) .Change following line in config.txt:
# Suppose you are running Privoxy on an IPv6-capable machine and
# you want it to listen on the IPv6 address of the loopback device:
# listen-address [::1]:8118
The last step is to configure the IE6 to use Privoxy which is running on your Windows 7 host machine as proxy for all addresses (including localhost)
And now you can use your Windows7 XP Mode IE6 to connect to your Visual Studio’s webdev web server.