It's rare that I do anything good in chess, and eventually these games are inevitably buried under the hours of dross I contribute to sullying the legacy of this fine game.  As such, I think it would be nice to have the opportunity to easily arrange our games archive by motif/theme/whatever else we choose, so that one smothered mate I did might be easily accessible.

I'll understand if this is really difficult or whatever.  It's my fault for being too lazy to work out how to do this with existing software, anyway.

For sure, the review options are limited here.  I always export the PGNs of my completed games and keep a summary archival PGN of my whole history that I can open in other more versatile programs.

Yeah, I should probably do that.  Thing is, this machine of mine is kind of old and I worry about its capacity to continually store new programs.

Is there decent software available that does allow grouping of games by theme and such?

Once it's a PGN file you can use a text editor to add whatever fields and values you want for each game, and even the small programs like ChessPad (which is what I use for simple reviews of my ongoing and archived games) allow the list of games to be sorted according to the various fields.  So if you developed a good categorization scheme for yourself and then correctly tagged your games, you ought to be able to retrieve what you want pretty easily.  

ChessPad is a pretty small footprint.  I've got a game open in it now and it's only using 8.3 MB of RAM.

I use ChessBase Lite (I admit, there may be better options out there), which not only allows you to manually tag games by theme, but also can automatically assign ECO codes based on the opening chosen. Furthermore, it comes with a free version of the Fritz chess engine, which is helpful when analyzing sharp tactical positions (after the game is over, of course). You shouldn't have any problem installing this on an older machine, either.