Hmmm...
I think I get it.
A little mome more stuff from my textbook just in case...
Basically, if it's a situation you have no idea which is better, you can use わかりません anytime.
While わかる/wakaru means to figure out the content of something or to be clear or to be understandable, 知る/shiru means to find out something or to become acquainted with something/someone.
* スペイン語がわかりますか?
Supeingo ga wakarimasu ka?
(Do you understand Spanish?)
* スペイン語を知っていますか?
Supeingo wo shitte-imasu ka?
(Do you know Spanish?)
知る/Shiru, with its basic meaning of to find out or to be acquianted, is translated as know when it appears in its ている/-te-iru form (lit., is in the state of having found out or having become acquainted). This is why you use 知っている/shitte-iru for knowing people and being familiar with places and ideas.
―カワムラさんの住所を知っていますか?
Kawamura-san no jyuusho wo shitte-imasu ka?
(Do you know Mr. Kawamura's address?)
―いいえ、知りません。でも、調べれば、わかりますよ。
Iie, shirimasen. Demo, shirabereba, wakarimasu yo.
(No, I don't. But I'll know (lit., it will be clear) if I check it.)
Both わかりません/wakarimasen and 知りません/shirimasen can be used for I don't know, but the nuance is quite different. 知りません/Shirimasen can imply I don't know and have no reason to, so it can sound a bit rude if the question is one that you could be expected to know the answer to. That is why, when you ask a store clerk a question about the merchandise, he or she answers with わかりませんね/wakarimasen ne instead of 知りません/shirimasen. (The ね/ne makes it clear that the meaning is I don't know, not I don't understand you.)
In fact, saying 知らない/shiranai directly to another person's face is a way of saying I disapprove of what you are doing and want nothing to do with you. Furthermore, since 知る/shiru refers to finding out something that you didn't have any reason to know, it sounds odd to use 知りません/shirimasen in reference to something concerning yourself, such as your own future plans or your own wants.
However, 知りません/shirimasen is a perfectly acceptable response when you don't know a person, are unfamiliar with a place or idea, or have no expertise in a certain subject area.