From the title I assume you are dual booting - With what?
Have you had another Linux install, and you are replaceing or is this the first one.
Have you used a partitioning tool, such as Partition magic, and what did you do, did it EVER give you error messages on exit?
Are you confident on the quality of your disk, i.e. does scandisk under windows come out OK, you might want to defrag your Windows drive.
What are the exact messages, and have you visited Mandrakes website - I really can't recommend this enough, hunt around on forums, they have support e-mails for newbies, it might be worth your while.
Try it on the 5GB drive, that will be plenty.