There are a few ways that you could accomplish this. One way would be to use
Microsoft Search Server Express[
^] (currently the 2010 edition). Another option would be using Lucene.NET (but some people are disattisfied with this option as you have to write a process to index the documents yourself).
While researching indexing myself, I came across
Xapian[
^] which is possibly the best open search implementation I've come across - it's faster than Lucene.NET and supports more file formats; the down side is that it is written in unmanaged code, so you will have to use interop to use it.