Steps must be taken in advance. Laptops are different from desktops. They have proprietary drivers that need to be available for boot or bad things happen. For example the CD-ROM won't be there any more. Then how will you load anything?
What you will have is a plastic rectangle that keeps asking for an operating system you can't install.
First, create a boot disk.
Second, create another boot disk.
Test them both. Make sure the CD works.
Now for some alternatives to formatting your hard drive.Click here
for a site thats offering Windows XP recovery CD's to those Vista owners that want XP. The offer is good through July 31, 2009.Click here
For the Toshiba site specifically for downgrading Vista to XP. It's a source for the drivers that inevitably don't get copied, what rights you have for which version and so on.
Both sites will give you some important insight before you take the plunge.