"An error occurred mounting partition /dev/sda1 in directory /mnt/mnt/win_c2 failed (Read-only file system)
This sounds like you are having problems with mounting the CDRom drive.
Could you post a little more about this "used machine". Most PC's have an IDE controller for their hard drives. Many CDroms use SCSI emulation to work, and wind up with a /dev/sda device.
Mandrake 9.1 is old. You may be best advised to get yourself a copy of Mandriva 2006. It is available as a free download now. The later versions have much better hardware recognition, and many more drivers available for hardware. See the links on this site for 'Download Linux'.
Did you get 9.1 installed? I'm not sure by your post. If not, it sounds like you ran fdisk and formatted your 40 gig drive. Mandrake has those tools built in when you install.
A simple way is to just delete your partitions, boot from the install CD, and use the built in tools to partition, format and install. Mandrake will, by default create 3 partitions. 1. will be / (root) 2. will be swap 3. will be home. Of course you can set it up any way you like.
If you can not boot from the CDRom, (some older hardware does not support this option, or it is not enabled in the BIOS) please let us know. You may need a boot diskette to load a driver for the CDRom before you can install.