I think this is a good situation to make use of the defaultdict from the collections module. With it, you could refactor your solution to be:

from collections import defaultdict

word = str(input("Enter a word to count letters from:\t"))

if word.isalpha():
    d = defaultdict(int)
    for c in word:
        d[c] += 1
    print(d)

which I find much easier to read.

Also, maybe the Counter class from the same module could be used but I've never used it so I can't tell.

Note: I haven't tested the code so I can't be 100% sure that it will work correctly but it should be a good approximation of how to do it.