I had heard someone say that GTA IV can't be played without reinstalling. I did check it with regmon and filemon for any interesting registry injections. These what I found
Apart of Games for Windows Live, registry only slightly modified:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Rockstar Games\Grand Theft Auto IV]
"InstallFolder"="D:\\Games\\Grand Theft Auto IV"
"RGSC"="D:\\Games\\Rockstar Games Social Club\\1_1_3_0"
"RGSCLauncher"="D:\\Games\\Rockstar Games Social Club"
Modify above path to suit yours, save as gta.reg and import it... You may need to logout for environment take changes.
So afterward, you need to reinstall Games for Windows Live
run RGSCLauncher.exe for first time
run GTA IV
I'm not sure about where account saved. probably here:
C:\Documents And Settings\YOUR USERNAME\Local Settings\Application Data\Microsoft\XLive
C:\Documents And Settings\YOUR USERNAME\Local Settings\Application Data\Rockstar Games\GTA IV