As long as you have some unpartitioned space or a partition that is free to be deleted & reformatted, you're good to go. Start off by backing up any critical information as suggesed by someone else here - Just in case. In my experience, I've not had a problem losing information during a Mandrake install. During the install, you'll be given the opportunity to choose typical install or expert. The expert install will allow you to choose the size and types of partitions to create. The other creates and sizes them automatically. Mandrake uses LILO as the default boot loader - I've been quite happy with it
It handles your boot menu for you: You can change the default operating system if you want to or let it default to Mandrake. Your windows partition will be available as /mnt/windows while you're in Mandrake. If you happen to have NT/Win2K/WinXP installed, your NTFS partition will be available as /mnt/nt and will be Read ONLY -- but you can copy files to a linux partition to work with them.
Hope all installs well for you -- Good Luck