If you are already using Picasa, then you can use Tools-->Experimental-->Show tag as album.
I use that in Picasa quite a bit, and find it very handy.
The trouble with tags is that they are usually data attributes within a photo-management database, and if you get a wrecked/corrupt database (as has happened to me once with ACDSee and twice with Picasa), then you can lose hours of work invested in building the data in the database. They are not usually easy to recover intact. So don't rely on the resilience of the database, because they are probably not all that resilient.
My workaround: Instead rely on the data you can store within the image file. If you have mostly .JPG files then you can have EXIF data for any camera-related data, and IPTC info. for any freeform data you might want to attach to the image. When you add a caption in Picasa, it goes into the IPTC info, and is immediately searchable. So you can put in or duplicate your "tag words" into the IPTC info, and then never worry if you lose their external tags if your database gets corrupted (because you can easily recreate the tags and tagged photos using a sort).
I therefore recommend the principle of having the photo records contain their own meta-data not only as a good data-security measure, but also because it reduces your dependence on the idiosyncrasies of any particular database system. So you can use Picasa, ACDSee, Irfanview, or whatever, to utilise the meta-data.
Hopes this all makes sense or is of use.
