When creating a multiboot system with Windows XP and Solaris 10 you need to install Windows XP first. If Windows is already installed, repartition the hard drive to allocate sufficient space for your Solaris installation. This should incude enough space for the OS (5Gb is recommended) and whatever extra space you anticipate you will need for applications and user files.

Once Windows is installed and the hard drive is repartitioned, start Solaris 10 installation. Solaris 10 will install a boot loader, which will start up befor the Windows boot loader and will allow you to select the OS to boot (either Windows or Solaris). If, after installing Solaris 10 you need to reinstall Windows, it will likely overwrite the Solaris boot loader with its own MBR (Master Boot Record) and you will no longer be able to boot into Solaris.

There are two ways of dealing with this problem:

1) The best way is to reinstall Solaris boot sector that was overwritten by the Windows installation. Here is an excellent tutorial by Mariusz Zynel that covers various issues of creating and running a multi-boot system with Solaris, Windows, and Linux (PDF, 204Kb):

Multi-booting Solaris and other operating systems

2) Configure Windows MBR to allow you to select Solaris. Place your Solaris main installation CD/DVD in drive D: (or whatever your drive letter is) and at the Windows command prompt type:

copy d:Solaris_10ToolsBootusrlibfsufsmboot c:bootsect.sol

Then open the C:boot.ini file and add the following line at the end of the file:

C:bootsect.sol="Solaris"

This method is preferable if you have to reinstall Windows on a regular basis or if you want Windows to manage your boot options.