Trim excess fat from the roast and put it in the slow cooker.
Break the stems from the guajillo chiles and shake out the seeds. Rinse the chiles and put them in a pot. Add 1 cup water and boil 5 minutes, or until soft. Drain and discard liquid.
Put the chiles and everything else in a blender and puree.
Pour this over the roast.
Cover and cook 10 hours on low, or 5-6 on high or until beef is fork-tender.
Shred the meat with two large forks and mix with the sauce.
Notes
If using chili powder rather than whole chiles, skip the boiling step and just put everything in the blender together.