〜てある: a state left by someone's action
A window being open and someone having left it open have different emphases. Read transitive verb + てある as a state remaining from a deliberate action.
From a transitive action to its result
窓が開けてあります。 describes a window that someone has left open. Add あります to 開けて, the て-form of 開ける. The emphasis is on the remaining result, rather than the action of opening it right now.
予定が紙に書いてあります。 says that the schedule is written on paper. It can describe the result of a deliberate action without naming the writer. Do not translate it as an action in progress, like 書いています.
Comparing 開いている and 開けてある
窓が開いています。 describes the window as open, using the intransitive 開く（あく）. It does not establish who opened it or why. 窓が開けてあります。 more directly presents the state as the result of someone's action.
Both sentences may describe the same window. An open window does not automatically require てある. Choose the verb and ending according to whether you are reporting a state or presenting it as something someone has arranged.
Distinguishing preparation from a present state
資料を印刷してあります。 can report that the materials have been printed in preparation, retaining を. Learn examples with が for the resulting state and examples with を for completed preparations, rather than changing every object marker to が.
印刷しておきます, with ておく, can state what you will do in preparation. Distinguish checking an already arranged state from deciding on a preparatory action to see how the two expressions relate.
Check: Does 窓が開けてあります。 mean the window is being opened right now?
Show the answer and why
No. It describes the state remaining after someone has opened it. To describe the current action of opening the window, use an expression such as 窓を開けています in an appropriate context.