If you haven't had your CRT shipped over yet, I have a solution. There is no software, to my knowledge, that adds scalines to PC games, but there are many pieces of hardware that add scanlines to an image directly through VGA. They supposedly create better results than software generators. Not having used one myself, I can neither confirm nor deny this, but a quick Google image search for "scanline generator" does seem to support this side. The cheapest one seems to be hardware enthusiast Toodles' T-SLG v1.6, available at his Godlike Controls website:
http://godlikecontrols.com/It seems somewhat ridiculous that software filters for old PC games do not exist yet, considering the PC community's tendency to mod and tweak. Their utility may seem a little less obvious to most -- after all, most older PC games can be made to run at higher resolutions without any more difficulty than modern PC games. Still, some games simply need scanlines to be playable, not just to look good.
Thief relies heavily on the player's ability to judge light values from a distance, but no light values look like they should on an LCD without scanlines.
Splinter Cell: Chaos Theory, aside from suffering from the exact same problem as the Thief games in its single-player campaign, had large portions of its multiplayer completely destroyed by the move to LCD, as it became impossible for anyone to remain hidden simply because most monitors couldn't display darkness sufficiently deep enough to hide a character. I hope somebody takes a hint, and soon.