Well ideally, before reinstalling XP, in Steam you would right-click the game, select Backup..., then you can select to split up into CD, DVD, or custom size files for burning, then it backs up the game, so all that is required for reinstall is get the Steam client, double-click the .exe(s). Very nice. Why would I buy it from the store then?
But in your case.. since you are only left with the .GCF files you just need to get Steam to recognize you have them. They are essentially everything you need. Additional directories for in-game downloads, configs, etc are created when the game is ran. I just re-installed CS:S, DoD:S, and HL2DM from Steam backup files, and all it really did was decompress to the .gcf files.
So I would try: keep the GCF files in their original locations in your /Steam/ directory, load the Steam client, then since they are not on your Installed games list, try double-clicking the game(s) from the Not Installed list. While it is expecting to download the required files, it might recognize that you have them and simply apply any additional updates or just move them to the Installed list. It won't download things that you already have, so it might just be a very small amount in order to have it fully installed. If it attempts to download the whole game (i.e. it's not seeing your GCF files), I'd stop it and try to figure a way around it in order to save your bandwidth. Or see if verifying the game's cache will move it to the Installed list (if the option isn't grayed out).
And yeah, I'm very happy with Steam (and Valve) for the most part. I didn't get on to Steam until late 2004 when I bought Steam Silver, but even at that point I have had ZERO problems with it. And it's very convenient and offers good deals on games. There is quite a few new and good games for purchase now since I last logged on, and even better is some Steam-versions of Starforce games have SF removed.
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