Are you want to test application only on different version of IE only? or fire fox ,etc also ?
See below approach i have taken to deal this problem.
Configure virtual machine on your PC. like VMware Workstation, Ms Virtual Server etc...
so now you do have 2 machine available with you and set below configuration.
1)Your Actual PC - Install IE 6 , Firefox 2
2)Ur Virtual Machine - Install IE 7 , Firefox 3
This will give you clear idea and provide ease in testing.