The TWAIN interface is supposed to be a uniform standard linking applications (e.g., Photoshop) and image acquisition devices (e.g., scanners). Unfortunately, some of the scanner manufacturers do not properly implement the standard, resulting in Photoshop freezes or crashes.
If you place a tilde (~) before the TWAIN plug-in's name, you disable that interface and resolve a common conflict between Photoshop and scanner software. To scan then, the workaround is to just use your scanner's software as a free-standing utility.
This offers the benefit of allowing you to scan, while simultaneously using Photoshop for something else. Remember that scanning through Photoshop directly, even if your TWAIN interface does work properly, means that you will spend all that time just staring at the progress bar while scanning.
The TWAIN problem is serious enough that Photoshop CS4 doesn't even automatically install its TWAIN plug-in. That said, if you prefer to scan via Photoshop, you can find the CS4 TWAIN plug-in inside the Goodies folder --> Import-Export folder --> TWAIN.plugin.