。
On Windows-family OSes, the administrator account corresponds to root.
Literal
Windows-type OS [in-では] administrator-account [subject-が] root [locative-に] corresponds.
A technical comparison. The suffix ~系 ('line', 'family', 'type') is very productive in Japanese for grouping things by type: Windows系, Mac系, 文系 (humanities track), 動物系 (animal-type). The verb 相当する ('to correspond to, be equivalent to') is formal/written and governs the に particle for what the subject corresponds to. The compound particle では is for topicalized location/context ('in/at X, [as for that context]').