A lot of people have problems with Vista and their current software such as in house built processes that aren't compatible so some of you might need to install XP on a machine that came with Vista.
Warning: There are rumors some computer manufactures will void warranties if you "downgrade" your operating system. Be sure to check with yours before you do anything.
Windows Vista installs much different than Windows XP. The way Vista installs causes XP issues when trying to install it. The only way to install XP is to completely remove Vista and the XP format wont do the trick.
There are many disk cleaners out there but Darik's Boot and Nuke is free and works great. You don't need to do a DoD wipe, a simple one pass should do the trick.
Once done you should be able to install Windows XP with no problems!