This is really ridiculous, as it seems Microsoft (rude words here..) big time! If your GTA 4 is not running anymore, download and install the Windows Live Client from here: http://www.xbox.com/en-US/LIVE/PC/DownloadClientWindows Live Client.
This solved it for me.
This is not the first time Microsoft f*****d something up regarding software, without indicating what actually the problem is.
