I just reinstalled TW2 because I was having trouble installing the 1.2 patch. So I uninstalled the game. Deleted TW2 folder. Reinstalled the game, it automatically patched me to version 1.1 however now whenever I try to install patch 1.2 I get the "installed game version is invalid or incomplete" How do I fix this?

Make sure the launcher is not running when you install 1.2.

That should fix it. If not, remove any mods or dlc from you CookedPC folder.
I would guess the mods and dlc doesn't apply since you are starting fresh. So, hopefully step one works.

Had the same issue,and i solved doing this steps:

Remove all mods,redownload the gog patcher for 1.1 and run it again,let it finish and when the launcher repop it should present you a msg for the 1.2 ,press yes and let it do its job.

That is cool, but the message OP is getting means either the launcher is interfering and/or he needs to run the installer as administrator.

Yours maybe is set to run as admin by default.

OP could start all over and make sure always to install as admin or turn UAC to lowest and would likely get your results.

That would be why launcher installs for you. Most people don't run UAC at lowest, so they will run into problems sometimes.

I get the same error when I try to repatch 1.1 Also my UAC is turned off so its not that. This doesnt work either I get the same error.

Try deleting all the DLC from the CookedPC folder of your The Witcher 2 directory.
Those files are:
alchemy_suit.dzip
dlc_finishers.dzip
hairdresser.dzip
magical_suit.dzip
merchant.dzip
roche_jacket.dzip
swordsman_suit.dzip
troll.dzip

Aftwerwards, run the patcher_1.2.exe again.

I tried this and I get the same error and the dlc is installed again after I run it.

Have a look at your witcher2.exe's version number (mouse-over tooltip of the .exe file) and let us know what it is. The file is located in \The Witcher 2\bin\.

I just reinstalled the game again so im back to 1.0. How do I update now? If I try updating using the 1.1 patch from gog I get the same error.

Ok, now THAT is weird. Did you delete any left-over files / the game directory along with everything except savegames from the Documents folder after uninstalling the game?

Yea I deleted the whole Witcher 2 folder. I just went back and deleted everything in TW2 folder in my documents but saves and I still get the error.

I just read this on the official site:

If you are installing the game for the first time or re-installing it, the game will automatically patch online to version 1.1 during installation

So, what I'd try is
a) re-install the game again and immediately afterwards check the version number. Then, apply patches accordingly
OR
b) disconnect from the internet, re-install the game and try installing gog's 1.1 and 1.2 while no internet connection is established.
Sorry, I'm putting you through the installation process again, but I don't know what else to do, since we can't figure out which version installed and what happened while trying to apply 1.1 without success.

Important note: Never start the game launcher between installing and patching it to the newest, as we then get another uncertainty with the launcher downloading and installing (or not doing so) any patches.

I tried a) which results in me being in version 1.0 currently. If I try to install patch 1.1 after reinstalling the game I still get error.
I tried b) b as well when I dced my internet, but I havent tried installing patches while no internet is active. iLL give this one last try.