Error #1: "This patch should not be applied to this version of Dawn of War"
You need to have all previous patches installed before attempting to install 1.3. Going from a fresh install that means you will have to install 1.01, 1.1 and 1.2 before patching to 1.3. You can do this by simply logging on to gamespy, it will download and install all patches in the correct order.
If for some reason you want to download them manually, you can get them here. Watch out for different versions, if you have an English version of DoW you have to install the English patch, not the international one.
Error #2: "Installation of Dawn of War cannot be found"
The patch looks in your registry for the path you installed DoW to. If you have reinstalled your OS since installing Dawn of War, or if you have moved your DoW folder after installing it, this key will no longer contain the correct information. To correct this, open your registry editor by clicking on "start", selecting "run" and then typing "regedit".
A new window will appear. Use the explorer-style menu on the left to navigate to this key:
HKEY_LOCAL_MACHINE\Software\THQ\Dawn of War
and check that the value for "install location" on the right actually matches the location of your DoW installation.
Error #3: "Your patch is corrupt"
Two possible reasons so far. First one is that the patch is actually corrupt, i.e. something went wrong during the download process. Download the patch again from a different location.
The other reason is a lack of space on your c:\ partition. The installer needs to unpack some of the game archives to modify them, and they are fairly large. According to THQ you need AT LEAST 1.43 GB of space for this. Try freeing up space on your C: partition.
If that is not possible you can relocate your temp folder by changing an environment variable as outlined in this thread. You can try some of the other workarounds mentioned there as well.
If none of this helps, check to see if somebody else has already started a thread on the problem. If that is not the case create a new thread in the technical assistance forum please.