Yes, it is possible to have multiple operating system installed on a
computer as long as they are installed on different partitions. Generally,
it requires some type of boot manger which allows selection of the operating
system you want during the boot process.
As an example if you have win98 installed on one partition and installs
winXP on another, the winxP will automatically add the boot manger and allow
you to select either operating system when booting.
Note: Any application installed will only be available in the operating
system it was installed. Thus if you install office it will have to be
installed twice to be available for both operating systems.
Typically, applications are installed on the same partition the operating
system is installed on. Quite a few applications will allow you to select
the installation location. It does not have to be the same partition as the
operating system. However, in most cases there are files installed to the
partition the operating system is installed on. Furthermore, most
installation will affect the registry which is also on the partition the
operating system is installed.
