Brocolli's right, when installing windows it automatically erases the MBR and puts a new one which doesn't point towards grub, leaving you only with windows when booting up
No matter what distro you've got, to fix the problem you're going to need to boot up your computer from a linux livecd. The Madriva CD should do (if you've still got it around) otherwise another Live CD like knoppix will work aswell.
Now, I've never installed Mandriva on my machine, but I have already accidentally screwed up my MBR. So madriva might have a program on their CD that can arrange grub. otherwise, here generic steps to getting grub up and working.
1. Boot up your computer from the live CD and get to a terminal (console) and make sure you have root privileges
2. Find out on what partition linux is residing and it's partition type (if you have no clue the command "parted -l" or "fdisk -l" can help)
3. mount the linux partition if it's not already mounted. to check is it is mounted, type "mount" and look in the list to see if your linux partition shows up.
Ex: fdisk -l showed me that my linux partition is /dev/hda1. So when typing mount I'm looking for :
/dev/hda1 on /mnt/hda1
/dev/hda1 on /media/hda1
If it doesn't show up then it isn't mounted. if it does show up then you can skip to step 5
4. before mounting a partition we need to create a directory as a mount point then mount it like so:
mount -t filetype /dev/hda1 /mnt/hda1
where filetype is the file type of your partition that you discovered at step 2 (for xeample : xfs or reiserfs are file types)
5. Now we need to mount a proc, bind dev, and chroot to the linux partition.
mount -t proc none /mnt/hda1/proc
mount -o bind /dev /mnt/hda1/dev
chroot /mnt/hda1 /bin/bash
6. If you are sure that your grub is properly set up, then you can go to step 7. otherwise, you can check the configuration using a text editor to open /boot/grub/menu.lst.
Ex: vim /boot/grub/menu.lst
7. type "grub-install /dev/hda", reboot, and it should work !