With mandrake, if you have snags with the bootloader (and afew other things), you can put the first disc back in, then at the prompt hit F1 for more options, and you should have a ''dos like"input prompt, if you then type "rescue" and return, you get the options of re-installing the windows bootloader.
When you do the installation, in the post install summary dialogue, you can click on "configure", then you can just check through what will show up. Normally you'd have to check that if you don't want to have the linux distro as the default OS.
But no, you won't have damaged the windows (very unlikely), but something's gone "pear shaped" wiht the boot loader install.
I dont recall if you get the option to re-install the linux bootloader (either lilo or grub) when you can do the windows one, or if you have to re-install the windows one then follow the process again to re re-install the lilo/grub
Erm, never dealt with raedon cards, but they are supported normally (as far as I'm aware), though if it's mega new/up to date, then maybe not.
To start graphically, if you have a text login, you can normally just type in
though I don't recall if you can do that as user or whether you'd have to do that as root?