I've received an email from the Rockstar Games webmaster
We are aware that a small number of fans are having problems running GTA IV on their PC’s and we would like to assure them we are working to help solve these as quickly as possible.
We would ask anyone that is encountering difficulties to contact their local technical support helpline for advice and recommendations. These telephone numbers can be found in the game’s manual.
There is also a regularly updated technical support page available on the Rockstar Games website http://www.rockstargames.com/support/ where there are many useful faqs and information on how to resolve some common problems being encountered.
So, Rockstar are aware of the problems, and will be on the ball trying to make things run smooth. They won't leave you twisting in the wind. Expect a patch in the near future.