Can you install Windows XP on Windows Vista and switch between them?
Yes. There is a process called Dual Booting, but it requires two hard drives. Just search for "how to dual boot windows vista and windows XP?".
Simply buy an upgrade version of Windows Vista switch your computer on and put in the disc and follow on screen instructions. Note: Although Windows Vista can run at different levels of the amount of RAM it uses you should have 2GB or more to avoid crashes or other problems with windows vista having less will down the performance of windows vista a lot.
You have to create at least one more partition for Vista. If you install Vista on the partition where XP is installed Vista will delete XP. To avoid such problem you have to have one more partition. When you do that just start the Vista installation process from XP and choose just created partition for Vista.
Can you use the upgrade DVD of Windows Vista Home Premium to perform a clean install on a PC that already has Windows XP Professional installed?
If you are interested in upgrading to Windows Vista you can find several ways to do so. One of the first things you must do is make sure your computer is compatible for Windows Vista and has enough memory. You can purchase a Windows Vista disk from a electronic store such as Best Buy. Insert the disk and select install when the prompt comes up. Follow the install wizard as instructed.
Not directly. You would need to format the hard drive and install Vista from scratch. Note that the very best computers that shipped with Windows 98 SE, Windows 2000, or Windows ME are scraping the very bottom of Vista's minimum system requirements. It may not be a very cost effective upgrade to install Windows Vista on such an old computer.