You normally would get the TWAIN driver from the manufacturer of the scanner.

If the mfr doesn't offer one, you might be able to find something from Adobe, although its not really their business to provide them.

There shouldn't be a charge for downloading from either of those, if you can find one.

There is an alternate interface called WIA that might suit you. Drivers I've loaded for various of my scanners offer Import from (scanner) TWAIN or Import from (scanner) WIA. The WIA interface works on the ones that show it (in my Photoshop Elements 2) but the WIA interface is a bit "clunky" compared to the TWAIN so I've almost never used it.

Both TWAIN and WIA are just "communication protocols" that allow the scanner/computer to talk to each other in a standard language; but for either of them to work the scanner driver must incorporate information about what the scanner can do and how the standard instructions are interpreted by both scanner and computer.

Vista killed a lot of perfectly good scanners since Vista blocks any "direct to hardware" commands. Everything has to go through a "protection layer" and that means that new drivers had to be written specific to Vista. Most older scanner drivers also were written for 32 bit systems (and programs), and scanner makers frequently have not produced 64 bit capable drivers for "legacy" scanners, so 64 bit WinXP also kills a lot of them. If your WinXP is the more common 32 bit version, you should be able to use an older 32 bit TWAIN driver - although I haven't looked at CS4 on 32 bit systems.

John