Index against a subset of rows? Is it possible to create an index against a subset of rows fulfilling a certain condition?

If it is possible how does one ensure the index is used?
Yes, what you're talking about is a Filtered Index.  It works just like you think, by creating an index off of a subset of data.  Below is an excerpt from the above reference:


  A filtered index is an optimized nonclustered index, especially suited to cover queries that select from a well-defined subset of data. It uses a filter predicate to index a portion of rows in the table. A well-designed filtered index can improve query performance, reduce index maintenance costs, and reduce index storage costs compared with full-table indexes. 


To your other question:


  how does one ensure the index is used?


A few things will dictate this index being used:


A well-designed filtered index correlated with 
A well-designed query


Obviously things like data distribution and many other factors play a role, but that goes with both filtered and non-filtered indexes.