To use the High-Res DLC without it overwriting your manually installed textures:
In C:\Users\***\Documents\My Games\Skyrim\
*Path may vary based on Operating System*
Open the Skyrim.ini in notepad.
Add the following under the [Archive] tab:
HighResTexturePack01.bsa, HighResTexturePack02.bsa,
After the Skyrim - Textures.bsa
It should now look like this:
[Archive]
sResourceArchiveList=Skyrim - Misc.bsa, Skyrim - Shaders.bsa, Skyrim - Textures.bsa, HighResTexturePack01.bsa, HighResTexturePack02.bsa, Skyrim - Interface.bsa, Skyrim - Animations.bsa, Skyrim - Meshes.bsa, Skyrim - Sounds.bsa
sResourceArchiveList2=Skyrim - Voices.bsa, Skyrim - VoicesExtra.bsa
Save the .ini file!
Then, go into:
C:\Program Files (x86)\Steam\SteamApps\common\skyrim\Data\
*Path may very based on Operating System*
and delete HighResTexturePack01.esp
then delete HighResTexturePack02.esp
Now skyrim will naturally load the textures as follows:
Skyrim - Textures.bsa - normally
HighResTexurePack01.bsa - replacing anything it can from Skyrim - Textures.bsa with files from this
HighResTexurePack02.bsa - replacing anything it can from Skyrim - Textures.bsa with files from this
And now you can use the High Res texture pack, in addition to your own manually installed textures. :3