okay, i guess i know what to do in this case! (ariel) . this could be the problem of a firewall you are using such as avast ,AVG,Mc affe and many more. there have been couple of cases, where avast shows there is a virus and avg blocking the application unless it is turned on( to prevent the access of inter net to the application).
things you got to make sure whether they are enabled or disabled:-
*just try ro remember whether the firewall or anti-virus was turned on during the initial installation.
*did it show any malware/virus/spyware/trojen?
things you got to do
*try to add an exception to the windows firewall( hope you know how to do this).run the app and see whether it's working .if it doesn't skip to the next point.
*if your firewall was active at the time of initial installation (first installation) uninstall VC:MP and turn the firewall down( stop it) and then re-intall VC:MP
*if your firewall was not active at the time of initial installation (first installation) uninstall VC:MP and turn the firewall up( start it) and then re-intall VC:MP
*if all these do not work ! just try to make a fresh copy of vice city( re-intall ) and do not MOD the game. install the client and run the application then.
hope your problem will get rectified! .post your experience here