For anyone with this problem, please try the following.
- Install the SA-MP 0.3c R3 client
- Download this zip containing a replacement samp.dll
http://www.sa-mp.com/stuff/samp_dll-03cR3-intel.zip
- Extract the samp.dll from the above zip file and paste it in to your GTA San Andreas folder, replacing the existing samp.dll
- Run the game and see if the chat screen works as expected
- If it works, please report so here
- If it doesn't work, please post the contents of your chatlog.txt, found in your GTA San Andreas\SAMP folder under My Documents. This file should contain the reason why it didn't work
Thanks