weak at the knees
1. Literally, in danger of collapsing because one's knees may buckle, due to faintness brought on by a strong emotion, exhaustion, etc. Hearing the news of my father's sudden death, I went weak at the knees and could barely remain standing.
2. By extension, affected by a strong emotional reaction to someone or something. In this usage, the reaction is usually positive and associated with romance. Just thinking about Samantha makes me feel weak at the knees.