Can I install Solaris x86 on a system that already has MS Windows 9x/ME/NT/2K/XP (among other systems)?

To shrink an existing MS-DOS/MS Windows partition, if you need to make room,
use something like Partition Magic.

Problems have been reported inter-working with OS/2, however.
Reportedly, the order in which you install things is very important.
Solaris doesn't "share" computers and partitions really well.
If you have problems, you may need to install Solaris first,
on a partition towards the end, reinstall the boot manager
and add the partition.

Other options for booting is to use System Commander (see question below),
IBM's Boot Manager (bundled with Partition Magic or OS/2), FreeBSD's
OS-BS (see question below), System Selector,
or OS-BootSelect (open source), instead of the Solaris' Boot Manager.

Mariusz Zynel has a detailed information on booting Solaris with other
operating systems at

http://math.uwb.edu.pl/~mariusz/multiboot/

[Thanks to Barry Katz, Brandon Hume, Joseph A. Faracchio, Joelle Nebbe,
Eugeny Kuzakov, and Sean M. Kelley]