Rockstar Games are now officially giving support at our forums in order to help resolve the issues with Grand Theft Auto IV PC. To keep things organised, we have created a new sub-forum called PC Version Support, where specific troubleshooting issues can be posted for Rockstar to investigate. The main thread remains open for more general/less specific issues.
Before posting an issue, please check to see whether it has already been posted, and please remember to include as much information about your issue as possible (e.g. your PC configuration, where/when the issue occurs, your operating system, and which version of the game you have). Please try to be patient also, as there are a lot of issues to get through.