Installing Linux in an existing Windows Machine is not at all a big deal, rather it is quiet straight forward. You just have to leave some hard disk space for Linux and install it on the remaining hard disk space. The Linux boot loader is INTELLIGENT enough to identify that a Windows OS is already installed there and it will automatically adjust the boot loader "GRUB" for dual booting.
However our famous Windows operating systems are not that intelligent but clumsy and it will delete the whole boot loader "GRUB" completely. We will reinstall the GRUB and edit the /etc/grub/grub.conf file to correct it.
I want to go generalized and this how-to is applicable for any version of Linux using GRUB bootloader and windows os. However here I used Ubuntu 9.1 and Windows Server 2003 both 32 bit.