Hello Nirmalsingh, and welcome to Vista Forums.
It appears that there is no support for the Canon CanoScan LiDE 20 with the 64 bit Vista. There is however drivers available for the 32 bit Vista. Of course this would mean that you would need to do a clean install of the 32 bit Vista if you still wanted to use the scanner.
Canon driver download page:
Photo Scanners - USB Flatbed Scanner - CanoScan LiDE 20 - Canon USA Consumer Products
Hope this helps,
Shawn