That's an interesting point. She did have a giant snake all over her in that movie... So either she's not afraid of snakes and hamming it up and doing a really bad job here of acting like she's afraid, or she is really afraid of snakes and that scene in From Dusk Till Dawn was one of the greatest performances of all time.
"Salma Hayek has a real fear of snakes and had always refused to be near them. Naturally when she read the script, she knew her phobia would prevent her from taking the part. Robert Rodriguez conned her into thinking that Madonna was ready to nab the part instead so Hayek spent two months with therapists to overcome her fear."