In modern German both variants exist, each with a different meaning
- zurzeit: currently, at present
Example: Helga ist zurzeit nicht im Büro.
- zur Zeit: at the time of
Example: Zur Zeit Goethes war die Rechtschreibung anders als heute.
This was not always the case as before the 1996 Reformed Orthography both variants were allowed in the meaning of 1. above. Therefore you still may find the old spelling in books, or articles published before 1996.
Currently we have to build a composite adverb when the distinct meaning of their parts are lost (rule §39 of the Official Spelling Rules). In prepositional usage (rule §39 E2.2.3) we need to separate the preposition and the noun.
Also see: Canoo.net: Rechtschreibung von zurzeit