Make a special hard drive for testing.
Install the browser that you wish to use and then wipe the drive for the next browser.
IF you use Ghost or the equivalent, it will take 5-10 minutes max to recreate the partition when you need need.
Create a clean OS install without any browser.
Make a master copy of the partition.
Install the first browser version.
Ghost it to a unique place (CD or file name)
Restore the clean master copy
Repeat steps with each browser that you wish to use for testing on.
When you want to test a given browser, just restore the relevant ghosted image.