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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:
If the issue persists, the following instructions are provided as a guide to assist you in deleting the conflicting DirectInput drivers:
**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.
The patch promised by Rockstar Games last Saturday is now available. Start your game and connect to LIVE to trigger the download. The patch is 32.6MB, takes a few minutes to install, and contains a variety of fixes including the following:

Rockstar have stated on our forum that they are "going to keep supporting GTA IV PC for a long time," and that we can "expect additional fixes/improvements in the future."