Heyo, I may have an answer for you. Now if you can't read some of the stuff in this e-mail, you may need to install Simplified Chinese on your comp.
If they're from Aomeisoft, they're actually, really legit copies of the games, and you can register them and get a CD key. You can tell if they're Aomeisoft if they have a little blue label on the front of the box and the manual that says AEC on it.
You need to go to www.aomeisoft.com. Then look for the game -- you'll see a familiar picture, and there should be an accompanying link under it that says CD-KEY. If you don't see it, go to the menu at the top and look for the one that says CD-KEY. Then you pick the game from the list. Thank goodness the Chinese are adapting in so many English acronyms for computer stuff.
You'll have to fill in a form. Scroll to the bottom of the screen; it looks very much like a registration form, regardless of the language.
As it happens, I can read Chinese characters (albeit slowly and far from perfectly), so I had less trouble than you might. I'll try and walk you through this.
Get the manual for the game and look for the blue sticker stuck around the corner of the cover. It will have the CD-KEY申请号 on it. Scratch the little panel and get that number.
On the form, you'll find a bunch of stuff, but like most forms you can ignore most of it. Only the ones with "*" do you need to worry about.
The first starred box is "真实姓名". That means "full name." Put your name in there. The next box below it has 电子邮箱, that means your e-mail address. Sometimes they'll say this and sometimes they'll say E-MAIL. It's one of those things.
The next starred box is for your (full) address. The label reads "详细地址". The line below it has a box and a drop-down selector, you need to fill in both. The box on the left is your postal code (i.e. zip code) and the one on the right is the Chinese province or city district you live in. Well, you don't, so you'll want to pick the last one on the list, which is "其他" and means "other."
That CD-key should work fine, and it's perfectly legit (at least, the copies I've got are). You MAY have some problems installing the game, because it'll load up complete garbage instead of Chinese. If you can't read Chinese, well, setting your computer up to recognize the language properly on the install is pointless in any case, so don't worry about it.
I know with Half-Life 2 it will autoinstall in Chinese. To fix that, you need to go into the config files (outside the game) and set the language back to English. It's not critical. I don't believe any of the dialogue is in Chinese, but you'll have to put up with all the menus, substitles, etc. being in Chinese otherwise. I don't know about Doom III.
Hope that helps. If they're bootlegs, they're probably screwed up. Bootlegged software is notoriously crummy, especially when it comes to computer games.
Oh, word of warning: Valve and/or ID may or may not be willing to help you if something goes goofy. These are LICENSED legal copies, but that means that support is in Aomeisoft's hands, not theirs. I know this is the case with the Blizzard stuff.