So i just bought a 500GB HDD, now i want to use these three OS on my PC.

I know how to dual-boot XP with 7, but i'm new to linux, so...

BTW, would i need 3 partitions for these 3 Operating systems (1 for each) or can i install Ubuntu in one of those two windows partitions? (eg. C: XP&ubuntu, D: 7)

What would be the easiest & fastest to do? And how to do it?

Please answer ASAP.

i think you need to figure out the right order of installation.
Like:xp,7,ubuntu

Separate partitioning for Linux is a much better idea. There are some ways to install Linux inside Windows. I don't know the details and I think installing Linux in its own partitions is better.

You probably should have a small (maybe 2GB) swap partition for Linux, so four partitions total rather than three.

The beginning of a disk drive is faster than the end. So if you will be using one of those OS's or partitions a lot more than the others, you may want that one at the beginning of the disk drive.

It is simpler to partition a disk in the same sequence you install the OS's, so the OS you install first ends up at the physical beginning of the drive. But if it really matters, there are several ways around that, so the OS installed first may be physically wherever you want it on the drive.

When installing XP, 7 and Linux, it is simplest to install XP first, then 7, then Linux. But that also can be done in a different sequence if you prefer.

Standard MS MBR code transfers control to boot code in the first sector of the active primary partition. I assume the method you know for multi booting XP with 7 is one that preserves that design (selects the OS to boot after the boot code in the partition boot sector has loaded later stages of the bootstrap on one OS).

Standard (grub2) MBR code for new versions of Ubuntu transfer control directly to later stages of grub2 in the /boot directory of the partition where you installed Linux.

You can configure grub2 to offer one or more Windows choices on its menu and when those are selected, to chainload to the partition boot sector of that Windows install. Alternately, you can setup grub2 with its first boot sector written to its partition boot sector instead of to the MBR, then configure Windows to offer the choice of chainloading to grub2 (so the same boot menu system that lets you dual boot two versions of Windows can boot Linux as well). That approach is less common and less well documented but would work just as well as the common method of having grub2 as the main boot menu.

^ Thanks alot, i'll start the installation right away, if i face any problems, i'll ask you.

Regardless of the order Xp is booted by NTLDR incompattible with Win7 so it can't boot WIn7.

Win7 use bootmgr which can't boot Xp but can load NTLDR so that NTLDR boots Xp.

Both WIndows will claim the MBR in their installation. SO if

you should install Xp first

then Win7 and

Linux last then everthing will be done automatically for you by their installers.

A normal set up wil be Ubuntu dual booting itself and Xp but the installater may mistakenly give you the option to boot Win7 which should be unbootable by its own as I explain below.

When you boot Xp you should meet bootmgr menu with "older version of WIndows" as one choice and Win7 as the second choice. The former boots Xp. It is bootmgr that dual boots the two Windows and Win7 boot loader bootmgr actually resides inside the "c" drive belonging to Xp. Win7 partition should have no boot loader inside it as bootmgr fires it up Win7 by GUID reference.