Patch corruption fix #85469:
Had this problem too, with no mods or any alteration to the game.
Using File Monitor (a very useful utility), I noticed the patch was only reading a few files before declaring "itself" corrupt (very misleading error message). from the dawn of war folder ...
patch.err (empty file?)
And a few misc files from the %temp% folder ( start > run > %temp% )
First thing I did was delete patch.err, no go
Second thing I tried was replacing w40k.ini with the one in the .cab, progress! - now I'm getting the version error
Opened the .ini in notepad and changed the gameversion to 1.01, saved and ran the game once (since I assumed the game would need to fill out some things in the .ini), patch reports it's corrupt again.
So I go back to the DoW folder and nuked patch.err again, the dow\patch folder, and hopped over to %temp% and deleted everything there (might find some files are "in use" if you have lots of programs running, I ignored those)
Patch installed successfully
Replace w40k.ini with default from cab, edit gameversion to 1.01, delete patch.err & /patch, clear the contents of %temp% > successful patch install.
I haven't fully tested my game after trying this method, so at best consider it a step before total reinstallation.