Hi Doodle. So, have you successfully installed Mandriva and now want to get rid of it? Can you boot into it?
Normally, if you install Linux after Windows you get GRUB (the Linux boot loader) come up first and you can choose which OS to boot.
If that is not the case something has gone wrong.
If you can boot Windows then go into control panel>system administration>disk management. This should give you a graphic of your hard drive(s). The Linux partition will show as empty space.
So, say you see 100GB of empty space and it does not say NTFS, then this is your Linux install.
You can get rid of it by reformatting the partition as NTFS. Then you can use it for anything you like, say storage etc.
Hope this helps, John
PS. reformat by right clicking on the partition and using the drop down menu