Hi Kaushik,
You can download the latest kernel for your installation from below path:
service.sap.com/swdc --> Support Packages and Patches --> Browse our Download Catalog --> Additional Components --> SAP Kernel --> SAP KERNEL 64-BIT UNICODE
Here you will all different kernel releases. You can choose SAP KERNEL 7.21 EXT 64-BIT UC for your installation.
Make sure to choose correct flavor - Windows on IA64 64bit or Windows on x64 64bit
Please attach log files from work directory to see the error.
Regards,
Sharath.