First, the entire portion that you quoted that is not all modifier.

The relative clause is just "which keeps breaking down." The other portion is the predicate of the sentence. So the core of the sentence is "The PC is under guarantee until March. The modifier is "which keeps breaking down" the modifier begins with a relative pronoun and modifies the closest prior noun - "PC."

Relative pronouns do subordinate the relative clause to the main clause. The example that you mention above is an adjective clause because it modifies a noun. It is the PC that is modified.

A subordinate clause in the way that you are thinking of it is basically a sentence that is made dependent by use of a subordinate conjunction and in this way two sentences can be joined together. "He is your friend" is a sentence. But it can be subordinated so that you can say, "You should be nicer to him, because he is your friend.

So there are several ways to create a subordinate, or dependent, clause. And a relative pronoun is one such way.

Does that help?