I'm currently running windows 7 64 bit, and will very soon upgrade to windows 8.1.  What do I need to backup so I don't lose all my custom textures, models, maps etc.?

Well, depending on how much space you have, you could do what I did. I had to upgrade imidiatly because of some compatability issues so I didn't have time to ask a question. I took my entire WAW root directory, my Appdata mods stuff and put it all on an external HDD. Then I installed WAW and and replaced all the files In the root directory with the ones on my HDD. I also kept my Nazi_zombie_____.map files on another drive seperatly, just in case.

Good luck.

Thank you for your answer but I have got a more detailed solution by pm.
-copy raw and map_source folder (with copy I mean backup on a external harddisk or something like that)
-copy your map from root/mods/yourmap (for the scripts etc)
-copy gdt and files use with the gdt
(texture assets, model_export folders)
-install mod tools on new system
-reinclude those files in new installation
-test if the mod tools work then if u haven't already copy and paste the old map into it.

I'm just posting this for others who might come across this problem so they don't have to make another topic about it.
Credits for the solution goes to codmoddd1234