wikipedia is an ignoramus's best friend "
TWAIN is a standard ... programming interface ... that regulates communication between software applications and imaging devices such as scanners and digital cameras." Figuratively speaking your scanner speaks Spanish and your computer speaks English and the Twain driver is the bilingual translator.
Make sure you have the latest driver for the OS you are using. If you are running 32-bit Windows then you need a 32-bit driver, 64-bit OS needs 64-bit drivers. Always create a restore point before installing new drivers...
http://www.howtogeek.com/howto/3195/...-in-windows-7/
If you don't get it fixed please mention if you are running 32 or 64-bit?
And on that drivers page it offers software to scan for the right drivers. Always be wary of such software as sometimes they are malware installing bad things on your machine but most are likely to recommend a few incorrect drivers (at which point you may be needing that restore point to fix your computer).