So far it works on MAC but I am not sure about Windows but it could maybe work on Windows let me know if you have tried it on Windows and it worked!

These are the 5 Spectrum Camo files for the SIX guns it will be applied to. The guns with the skin will be the M4 Carbine, Mini Uzi, M16, Mp5, G36C (Not Shown) and all 5 of these files will be in localized_english_aw07.iwd. The M40a5 will be in localized_english_aac.iwd. 

In the Main Folder...

rename "iw_00.iwd" to "localized_english_aa.iwd"
rename "iw_01.iwd" to "localized_english_aab.iwd"
rename "iw_02.iwd" to "localized_english_aac.iwd"
rename "iw_03.iwd" to "localized_english_aad.iwd"
rename "iw_04.iwd" to "localized_english_aw07.iwd"
rename "iw_05.iwd" to "localized_english_aaf.iwd"
rename "iw_06.iwd" to "localized_english_aag.iwd"

Then using Betterzip or WinRaR replace the original gun files with the files you just downloaded (Spectrum Skins). Once you have done that save as a zip to desktop and then make the extension back to an ".iwd" and then replace the original localized file. Run COD4 and it should work.

PM if you have any trouble or questions!