I'm guessing it would need to be installed separately. Not everyone has a PC capable of running HD textures well, and it's a huge download.
Just make sure you have a 64 bit OS, that is the biggest issue. A 2GB or better video card is also recommended. Do look at the Unofficial Patch for the Hi Res pack, it fixes some issues.
if you go at the store within steam and write "skyrim HD resolution pack" at the search panel you get this: http://imageshack.us/f/600/qchy.jpg/
or you could go here as well: http://store.steampowered.com/app/202485/
It's free btw, but read the requirements first !!!!!
---------------------------
Easiest, most successful way to remove the HD packs is to disable them from loading in the Data Files section of the Skyrim launcher. The files are still on your hard-drive, but the game won't load them.
To actually remove them and not have them download again is to contact Steam Customer Support and get the files removed from your purchase list. Then you can delete them from your HD.
HAVE FUN !!!!!