A reply for your question
I am new to mandrake, but i have enough experience with RedHat 8.0.
It appears that your harddrive either overwritten the boot sector which caused the harddrive to "disappear" from your computer, or the problem is that you have changed some definition that you did not tell us about, if you can give me more inforamtion about the install you have done, please do.
Also what kind of partitions did you create?
How did you create them?
And what kind of partitions did you assign to linux?
And about the windows XP problem, it is possible that you have, by mistake, overwritten the entire partition and told redhat to use disk-druid, which removed all partitions from your computer and than installed redhat, which resulted in the removel of windows XP from your machine.
Please give me the information i gave you, it might help you.