Assuming, of course, that you have a 64-bit CPU (most modern CPUs are), it's easy to install both Windows 7 and XP Home (and many other OS's for that matter) on the same drive.
To truly isolate the two OS's, I'd use Boot-It NG, which is both a superb boot manager, and a partition management utility. Boot-It allows you to set each OS so it's invisible to the other OS ... although you can also have a common data partition if you'd like (I use this so all of my OS's see the same set of data). It also overcomes the 4 primary partition limit on MBR-based disks, which is handy if you decide to boot more than just a couple of OS's.
You can use it to (a) shrink your current XP Home partition to free up space for Windows 7; (b) create a partition for Windows 7; (c) create your boot menu to select the OS you want to boot; and then (d) install Windows 7 to the partition you created for that purpose.
It's a bit "geeky" ... but not hard -- and it's a rock-solid, stable utility for managing your partitions; imaging your OS's; and controlling the boot process. It also makes it trivial to later change the structure; add other OS's; remove OS's; etc.
As an example, here's a recent version of my boot menu (it now has 2 versions of Windows 7 on it, but I haven't updated my picture yet):
- Boot-Menu.jpg
- 22 KB
Boot Menu
by: LeeTutorPosted on 2009-08-13 at 19:03:01ID: 25094768
Well, first of all, you can only install the 64 bit version if you have a 64 bit CPU. If so, you can install it on the same drive, but it must be in a different partition of that drive. I don't know what you mean by "next to it without any interruption my current OS and system files." The new Win 7 partition won't bother your system files; it will only change the XP bootloader to use the Win7 bootloader and then give you a dual boot choice: either XP or Win7. This page gives a pretty good tutorial on how to dual boot XP and Win7:
http://lifehacker.com/5126