Some users are experiencing a "spinning camera" issue after installing the new GTA IV PC patch. Rockstar Games has apologised for this, and provided a temporary solution at our forums:

To fix the issue:

A1) Unplug any DirectInput controllers (gamepads, joysticks etc..)
A2) Reboot your computer
A3) Launch GTA IV

If the issue persists, the following instructions are provided as a guide to assist you in deleting the conflicting DirectInput drivers:

B1) Exit game
B2) Go to Start->Control Panel->System->Hardware Tab->Device Manager->Human Interface Devices
B3) Locate drivers for devices
B4) Delete drivers for devices
B5) Launch GTA IV

**Some users are able to reinstall the drivers for their DirectInput devices, with the device attached, and have found that it will work in game. If you do attempt to reinstall your device and the problem resurfaces, please follow steps B1-B5 again.

Related Link: Grand Theft Auto IV PC Troubleshooting Forum