I am not being able to access the partition which is used by Ubuntu. In fact I don't know how to mount that partition from Windows 8. Hope folks may help me.
Ubuntu is using another filesystem type by default called ext4 (resp. ext2 or ext3) which Windows cannot read by default. You need additional programms like Ext2Read to display the content of that partition in Windows. The other way around it is much easier because Linux kernel supports Microsofts NTFS by default. If you want to acces your Linux partition under Windows more frequently you might want to consider installing Ubuntu on a NTFS formatted drive.