It might be possible if you can find all the drivers. Start with the Toshiba web site and verify it's possible.
Why would you want to do this?

You can't do any "Upgrade" to an older version of windows. You'll need to format the hard drive and start from scratch, but Yes. At this point virtually any hardware that has Vista drivers has XP drivers (but not the other way around). Take Hughvs advice and go to toshibas site to download all the drivers in advance, then use the XP CD to format the drive and install a fresh copy of the OS.

Be sure to keep the Toshiba restore disk around in case you decide to go back to vista down the road, you paid for the license; you may as well keep the option.

yes. it is possible, but having 2 O'S ur computer will be slow, however it depends upon RAM. if it is 2GB RAM no problem to install xp.
what i suggest is go for ultimate version that is the best 4 vista.

Yes it is possible only but it will slow down speed but if you ram of 2gb
you should also visit the toshiba site

What edition of vista is it. If its business or ultimate, you could use downgrade rights.

If HP, Drivers are somewhere in C:\SWSETUP if I remember well!
Make sure you find and backup your systeme before do anything. NOT too far from what others have said though

Having two OSes on your system will not slow it down in any way. They must be thinking of "virtualization" where you install software that emulates a second computer, running one as the host and a second as a client. Simply setting up a dual-boot system with two seperate operating systems will not slow down your system except for the few extra seconds it takes to select the operating system to boot from.

That true!
Emulating is running two things at same time but pure multiboot runs one in one!
+1 vote to your answer

As others have pointed out, you are only legal if you have downgrade rights from Vista (Ultimate or Business, I think), or if you have an XP license from elsewhere (site license, full copy, ...).

On a technical level, you need to have your hard disk drivers available to the XP installer. Most modern systems need drivers that a plain XP install CD doesn't have. If you have downgrade rights, Toshiba may supply a suitable CD. Alternatively, you'll need to "slipstream" the drivers into a customised CD. The nLite program handles this very well.
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Joseph Wright