Device Boot Start End Blocks Id System
/dev/sda1 2048 52430847 26214400 1c Hidden W95 FAT32 (LBA)
/dev/sda2 * 52430848 976771119 462170136 7 HPFS/NTFS/exFAT
sda1 would be the system_backup_partition, sometimes called "PQSERVICE".
sda2 would be the C: drive.
sdb must be the D: drive . .
- usually this is sda3
but you have two 500Gb hdd's
what I usually tell people to do - is to erase & use that D: partition,
as almost nobody uses it - they don't seem to know that it is supposed to used to store your Data. ( pics, movies, music etc)
in your case , since you want to erase windows completely,
reboot with the live_cd & when the installer gets to 'where to install' - use the option to use the Whole Drive.
- usually *not* the 1st option, - it's a bit further down that list.
You will lose everything on the hdd - including the current Mint setup.
There are other ways, to have a more efficient partition setup, but goes with this method,
until you are more used to linux, or are prepared to wipe the hdd, a few times more, to get used to how the installer works.