I've done some more testing of FIFA RTCW 98 using Virtual Machine. I'm almost certain, it's using letter of a first CD drive it can find. I've added one virtual CD drive to my virtual machine. Installed a game from the first one (D:) - works no problem. So I removed the game and everything what was left on a hard drive from it. "Ejected the disk" (virtual) and "Put it" into my 2nd virtual drive (E:). Tried starting a game and got "Please insert CD". And so I went to a device manager and disabled D: drive - the same. I then removed the game and installed again from the same drive (E) - basically an identical situation, but now installer couldn't find D: drive. I ran the game again - worked. So basically installer is witting somewhere a file that tells the game to which drive letter it should refer (registery is literally empty) - the only thing in it is "DisplayName"="FIFA RTWC 98", "3D Card"="None" and "Language"=dword:00000001.
After installation there is a INSTALL.DAT. It's only 2 bytes long. I have 2 versions of it - one from an installation I've made from D drive, the other one from installation from E drive.
From D drive it goes: 0104
From E drive it goes: 0105
I'm guessing this is what I've been looking for.
-- EDIT --
I've replaced 0104
with 0118
, which I assumed was going to be X and the game is now working on Windows 7. I guess it's time to write a page, then.