I have to agree with Margaret. I don't think you "need" one of those programs for your child to learn early on. My now 4 year old knew all his letters by age 2, by sight and sound, his colors, his shapes, numbers through 20....all this came from our reading together and play. He recognizes many words that are important to him, like our names (as a former first grade teacher I can tell you that names are very valid as "words"). Again, this is from reading together and playing with things that involve letters, shapes, etc. He also knows how to count to 10 in Spanish, and I credit a fun song we sing and Dora! Dora gave him the interest in Spanish so she does get some credit.
All in all, I think something like that wouldn't necessarily be harmful, and probably does work to an extent, but why do you need your 18 month old reading Charlotte's Web? The most important part of reading really is comprehension and a small child who is actually able to read those words does not comprehend that level of book when he/she is doing the reading. It's "word calling," really. They can understand what is going on when Mommy is doing the reading, though, and it shows baby that reading is fun, interesting, and something you can do together.
As a former first grade teacher (and first grade is the big "learn to read" year) I wouldn't necessarily push word recognition/word call. The love of reading and comprehension are so much more important. Your child's greatest learning really will come from your play together and your reading to her/him. Good luck!!!