I didn't make it!
I uninstalled EVERYTHING, reinstalled cod then the patch. patch says pak0.pk3 is not a valid previous version & could not be upgraded. I did it cause I could not download any more mods cause pks gave issues every time.
Did you delete all COD folders after uninstall as well? Is this a legal copy? A quick search says there are different versions of the patch and to install them in order.
I did a search in google with this: pak0.pk3 is not a valid previous version This will also be moved to the COD forums. Good luck.