I have W XP and I want to install also Centos.I have a pc with 60 GB HDD, 756 RAM, 1,8 Ghz processor amd athlon xp.. i want to know how do I partition my drive.I'll create a swap part, an ext3 partition for system but I want to know if it is necessary to create a new partition for boot loader.

Regards

you can just include the bootloader with / and just make a parition for swap and /

A seperate boot partition is not essential. If you have multiple distros it can be a good idea to use a small partition with just grub/lilo on it, keeping it independent of any particular distro. However, when you're just starting out, or you only have one distro, a seperate boot partition is of little benefit.

However, a seperate /home partition is a good idea. This will let you change/upgrade your distro without touching your personal data.

Thanks for your answers. My last question is where should I choose to install boot loader ( MASTER BOOT RECORD /... First sector of boot partition).I want to find a way to avoid overwriting boot loader after reinstalling windows.(my w XP works fine now but i never know when I have to reinstall my OS)

The Windows boot loader cannot (AFAIK) boot anything except Windows. So, you'll need to install to the mbr.
Obviously, re-installing Windows will overwrite the mbr, there isn't really any way around this, so you'll need a boot floppy or live cd to reinstall lilo/grub.

For example, you could make a grub boot floppy that would re-install itself automatically by making a menu.lst/grub.conf like so:

timeout 5

title Install HD
root (h0,1)
setup (hd0)

Obviously, replace hd0,1 with the appropriate drive/partition (see grub docs if you're not familiar with the syntax). You don't need anything else on the floppy (kernel etc), just grub. Then whip out the floppy and reboot.