Since you have photoshop cs5 in the x86 program files, then you have both the 32 and 64 bit versions of photoshop cs5 already installed.
the 64 bit version of photoshop cs5 is in
C:\Program Files\Adobe\Adobe Photoshop CS5 (64 Bit)
and the 32 bit version is in
C:\Program Files (x86)\Adobe\Adobe Photoshop CS5
If you haven't changed anything in the programs menu, then both versions of photoshop cs5 should be listed under Start>All Programs
Photoshop CS5 is the 32 bit version
Photoshop CS5 (64 Bit) is the 64 bit version
You put the Twain plugin in Win32 folder from the link listed in post #1 into the Photoshop CS5 32 bit Plug-Ins>Import-Export folder
C:\Program Files (x86)\Adobe\Adobe Photoshop CS5\Plug-ins\Import-Export
TWAIN plug-in | Photoshop CS4, CS5, CS6
Vuescan that Ronald Keller mentions is also a good option if you don't want to be limited to using the 32 bit version of photoshop cs5 to scan into photoshop.
VueScan Scanner Software for Windows, Mac OS X and Linux
Photoshop cs5 should also be able to use WIA support for your scanner.
That would be under the File>Import menu in photoshop cs5 after you have your scanner connected and turned on