If you are booting more than two operating systems, it's called multi-booting.
Assuming Window is already installed, the first step is to backup the Master Boot Record (MBR) and partition table. If you don't do this, you must perform the sacred rituals to appease the Multi-Boot Installation Gods. There are several tools available, one tool that has many many such useful utilities is:
(scroll down or search for Partition Tools)
If you don't like that one, google: save restore mbr partition table
Now that your MBR and partition are safe (unless you had no operating system already installed), you can follow the advice of tamoneya. Estimate your disk usage, and plan your partitions.
Essentially, how many partitions will you need, how big should they be and what goes on each one. There is also the detail of what filesystem to use for each partition. Here are some things to consider:
* Making an NTFS data partition for WinXP, so that you can put all your valuable data on a partition separate from the Windows system and applications. That way, if Windows crashes, you can reinstall it (after saving your MBR and partition table, of course, since a new Windows installation will overwrite the MBR) without losing any valuable data. This also makes it easier to manage your data.
* Consider making a small /boot partition, and figure out whether the 1024-cylinder boot limit
affects you, as this will affect where you should put it.
* Suse and Mandriva will be able to share a swap partition between them.
* You will want to share other partitions if possible. You might consider making an appropriately sized FAT32 partition to share data between all 3 operating systems. If you just need to share between Mandriva and SuSE, use a Linux filesystem instead, such as ext2, ext3, ReiserFS, or whatever else they both support well.
* If you have two hard-disks, you get more space, but it also makes multi-booting slightly more complicated.
Anyway, install WinXP first, making sure to limit the size of its partition. If it is already installed and taking up too much disk space, use the correct utility to resize it. Another warning: be absolutely sure
to have all your valuable data backed up first. Once WinXP is installed on an appropriately sized partition, then install whichever of SuSE or Mandriva you prefer to next. I would choose to install first whichever one I thought myself most likely to keep. The one you install last is most likely to be the one that overwrites the MBR last.
As a last step, once everything is installed and you are a happy camper, backup your MBR and partition table again, to protect yourself from future mischief. You should do this everytime you create new partitions or change your MBR.
Have fun, make backups first, and be patient.
Coffee and sympathy help.