I recently wrote about a way to run IE 6 alongside IE 7.  It may not be the best idea.

From the Microsoft IE blog:

If a user has ever attempted to run IE7 Beta1 in an unsupported side-by-side configuration with a version of IE6, IE7 Beta1 puts a registry key on the machine the first time a user executes the IE7 version of IEXPLORE.EXE. This key is part of an normal IE7 installation on XP, and will not be configured correctly if an unsupported side-by-side install is used. When IE7 is installed using the installer, the key should be removed properly upon uninstall. A machine can also load this registry key and not remove it during a failed IE7 installation.

Link here via SpywareSucks.

SpywareSucks makes this comment:

When I asked him why he wanted to run IE6 and IE7 together, the only reason he gave was that some sites do not work with IE7.   He was surprised to discover that this is an issue that is easily addressed, without running the risk of causing problems for your PC (such as those being reported to the IE team).  Advice and instruction on how to get around User Agent String sniffing, which is in 99.9% the only reason you may see problems trying to access a web site when running IE7, can be found here:

Alex Eckelberry