Efficient way to index text files

Funcionality for indexing text files is built into Windows Vista/7 and available for XP by downloading either Google Desktop Search or Window's Desktop Search.

Of course, if you're interested in only indexing and tagging these specific files, I would recommend Microsoft Office OneNote, it is web accesible and offers a lot of useful note-taking features.

With Microsoft OneNote 2010 you can collect and organize everything you need—notes, photos, videos, and web links— for any project you are working on. OneNote keeps the information you need at your fingertips and, with powerful search capabilities, you can easily find exactly what you are looking for. Toss those paper-based notebooks in the recycling bin—you’ve got a digital notebook now!, part of the Microsoft Office suite.

you can use:

• Omega, which is a preconfigured indexer based upon the Xapian search engine.
• Solr, which is a full text search server, based Lucene.
• Nutch, which is a search engine, also based upon Lucene.