Place dried guajillos in a medium saucepan over medium heat with enough water to cover. Simmer for 10 minutes or until chiles soften.
Remove from heat and set aside for 5 minutes. Do not throw away water.
Add softened chiles, tomatoes, and 1 cup chile water to a blender. Blend until smooth. Set aside.
Heat olive oil in a 5-quart saucepan over medium-low heat. Add fideo pasta and stir constantly until pasta is golden brown. Do not burn. Remove from heat and transfer to a separate bowl. Set aside.
Return saucepan to medium heat. Sauté onion and garlic until tender, about 1 minute.
Add guajillo mixture, oregano, salt, and pepper. Bring to a boil. Add fried fideo. Reduce heat to low. Simmer for 15 minutes or until liquid has been soaked up.
While fideo is cooking, prepare taco shells. Heat vegetable oil in large frying pan over medium heat. Test the oil temperature by dipping the edge of a tortilla in the hot oil with a pair of tongs. If the tortilla bubbles rapidly, the oil is ready.
Using tongs, carefully fry tortilla in hot oil for 5-7 seconds on each side. Fold tortilla in half using tongs. Fry each side of folded tortilla for 10 seconds or until crispy.
Lift taco shell over the oil, letting any excess oil drip off. Place shell on a paper towel-lined plate to absorb any excess oil. Repeat with remaining tortillas.
Scoop fideo in taco shells. Add avocado slices, a sprinkle of queso fresco, cream, and cilantro leaves. Serve immediately.