*This is an example for running Call of Duty 4 in patch 1.6 and 1.7
There's really 2 ways of getting both versions.
1. When you install CoD4 patch up to 1.6, copy that folder and rename it. Then update the folder you copied to 1.7.
2. If you have installed CoD4 and are already at the 1.7 patch, reinstall the game into another folder and only patch to 1.6.
Ok so I am assuming that you already have CoD4 installed on patch 1.6
Now you need to copy that folder and paste it (any where really) but you should probably keep it in the Activision folder.
You will have to rename one - What I did was a kept 1.6 named Call of Duty 4 - Modern Warfare and named the other folder Call of Duty 4 - Modern Warfare - 1.7
Now that you have 2 folders with the exact same thing you will need to patch one of those folders.
Download and install the 1.7 patch - When you are choosing the location pick the Call of Duty 4 - Modern Warfare - 1.7 folder and this will update that folder only.
When that is done updating you need to copy the iw3mp icon.
Go to properties and change the location to the iw3mp in the 1.7 patched folder.
When you want to run 1.6 you click your old icon, when you want to run 1.7 you click the new icon.
Insert your disk or whatever you use to install CoD.
Change the folder location to something other than the default location - Any folder but I recommend Activision and rename to whatever you like - (Call of Duty 4 - Modern Warfare - 1.7)
Install the game to that folder, install all the patches to the desired patch. You can do this as many times as you want. You could even install ever patch for the game.