Its called dual boot.   Google 'dual boot' for ideas if no one else replies.

Yes, although it is more complicated than installing XP first and Vista
second.

Install Windows XP On A Machine Already Running Windows Vista (MVP John
Barnett) -
http://vistasupport.mvps.org/install_windows_xp_on_machine_running_vista.htm

Malke

Yes, it is, but you should post your question in a
Vista newsgroup because you're going to modify
the Vista boot environment.

    Oh, yes, it's certainly possible.

    Normally, the older OS must be installed first unless you wish to
acquire and use some 3rd-party partition and boot management utility.
(In which case you have to follow the instructions provided by whatever
3rd party solution you select.) However, this KB Article (not for the
faint of heart or technically-challenged) explains how to repair the
Vista boot process after installing WinXP:

Windows Vista no longer starts after you install an earlier version of
the Windows operating system in a dual-boot configuration
http://support.microsoft.com/kb/919529

    MS-MVP John Barnett's Guide is considerably more user-friendly:
http://vistasupport.mvps.org/install_windows_xp_on_machine_running_vista.htm

Before proceeding:

    First and foremost, if the specific computer model in question was
designed specifically for Vista, there may well be no WinXP-specific
device drivers available to make the computer's diverse components work
properly.  If this proves to be the case , installing WinXP in a virtual
machine would be your best option, anyway.  Consult the computer's
manufacturer about the availability of device drivers.  Secondly, adding
another OS might void any support agreements and, sometimes, even the
warranty.  Again, consult the computer's manufacturer for specifics.

    Dual-booting is no longer necessary in a great many situations. Why
not download a Virtual Machine application, such as Microsoft's
VirtualPC 2007 (http://www.microsoft.com/downloads/search.asp?) or
Innotek's VirtualBox (http://www.virtualbox.org/) and run Win2K and your
legacy applications within a virtual computer.  Both are free and work
with Vista.

NOTE:  Microsoft does not support the use of VirtualPC 2007 on Vista
Home editions, but several people have reported that it works.  Your
results may vary.

EB added these comments in the current discussion du jour ...

yes, using Boot Manager