Go to Radio Shack or an electronic store and pick up a PS2 to USB adapter, yep you read correctly, a Playstation2 controller to USB adapter. I picked mine up for $12 at the Radio Shack in town and I have also seen them online for $10-15.
Then snag a copy of SAAC (San Andreas Advance Contoller) and install it. I found it thru a google search and at www.thegtaplace.com, there are also custom .ini files available that pre-load the controls specifically for San Andreas. In a nutshell, by getting the adapter and SAAC, it maps the controls for San Andreas on the PC exactly as they are found as if you were playing on the Playstation console. Believe me, this is the best way to play San Andreas on the PC. Sorry, I am not able to provide the links a this moment.
The only other option is to map the controls yourself in the game, but that causes issues when doing sub-mission, drive-bys, and flight controls.
P.S.: If you PM me and give me an email, I can hook you up with the SAAC and .ini file.