。
She had absolutely no interest in boys.
Literal
She [topic-は] boys [in-に][contrast-は] entirely interest [subject-が] did-not-exist.
The composite particle には does double duty here: に marks 'toward / in' (the target of interest) and は adds contrastive emphasis — 'as for boys (specifically), there was no interest.' The は often implies a comparison with other things she might have cared about. 全く~ない is the strong-negation pattern ('not at all, absolutely not'), pairing the adverb 全く with a negative predicate. 関心がある ('have interest') is a set noun + verb collocation; the noun 関心 takes が rather than を.