Yes it is possible to install multiple versions of linux on a hard drive, hec you can even make them use the same partitions for home and tmp etc.... but i wont get into that here. The decision your gonna need to make is when you install them, which one would you like to install the bootloader... and then edit the lilo.conf or grub.conf accordingly... if you dont know how to edit those conf files then just let the distro you install second install its bootloader over the one you installed first... it should give you and option to choose between both distros.

Yes. If you look at the www.tldp.org website, there will be some howto's on how to do it. You will want to update lilo or grub on one of the two distro's do that you don't get a case of the dueling distro's. You could also use the same /home partition for both distro's. If you use different user names, there should be no conflicts. Bear in mind that Mandrake starts the UIDs at 500 while SuSE uses a different range.

One way to do it is to use the root partition, rather than the MBR, for lilo or grub on the second distro, and have the first distro chainload the second's boot loader. This will have the disadvantage of going through two boot menu's to start the second distro, but this way, you can update lilo/grub on the second distro normally, making it easier to maintain.

Another way would be to use the XP's NTLDR to chain load whichever Linux distro that you want. There are numerous messages on this site on how to do this (use the search function) as well as numerous how-tos on the Net (use google).

Good Luck.

FWIW, my computer was running XP and Mandrakelinux 10.1 and I installed the SUSE Linux 10.0 retail (boxed) version using all defaults (except for time zone, etc.) It set up the bootloader (GRUB) perfectly and I can now boot into any of them.