1-Run the standard GTASA installer as administrator on XP SP2 compatibility mode (you can change these options from Properties) and then install the game.

2-After the installation of the game has finished, right click the whole GTA San Andreas folder, select properties and then uncheck the Read only box (disable read-only mode).

3-Run the TTDISA installer as administrator on Windows Vista SP1 compatibility mode;

4-Do the same as Step 1 with the GTASA.exe.

Notes:

I did not do step 4 and everything is still working fine on my Windows 7 X64 installation. If it does not work for you, try again from the beginning. This did not work the first time for a few but worked fine after they tried again.

The problems are caused because the TTDISA Mod installer was designed and expects to be run on Windows XP SP2 I think.

Credits for Windows 7 installation instructions goes to AlexLB on the Gtaforums

http://tinyurl.com/33n6q9f