Need more details - What’s the make/model of your computer etc etc
Chances are you have tried to download vista to an old computer that doesn’t have the memory or the processing capacity to handle the OS. You should of recieved a live CD that came with the computer containing XP? You will most likey have to make a backup of all your files and essentials from your computer and then format the HDD and reinstall XP. Make a backup of all files etc etc, defrag your computer, restart the computer and when the BIOS screen comes up, go into the options (Usually F8 or something like that, allthough will depend on your computers manufacture) in the BIOS setup you will need to change the boot setup so that it boots from the CD/ROM drive before it boots from the HDD. Exit and save your changes. Boot up the computer with the live CD containing XP OS, it should then take you to some sort of wizard for XP. Set it up then copy all of your backup files back to your computer, then you should be back to where you started.
PS - This is just a suggestion, seeing as you’ve messed everything up, you may want to consider the option of something called “open source” Basically There are many different and superior operating systems out there asides from Windows. Try googleing Unix or Linux for more information. I run Linux and have found it to be alot easier than windows as I can control everything from the commandline which gives me more control over the functions of the computer (if your not good with commands then versions of linux have something known as a GUI or graphical interface - similar to windows point and click system.) Just a thought but all the programs and applications are free and also the source code is also accessible meaning that you can customize everything to suit your own tastes.
I belive that ubuntu 8.0 hardy heron is the latest release of linux (check it out @ http://www.ubuntu.com/ ) You can even run it alongside Windows or as a dual booting system if you don’t wanna go cold turkey!!!
- Hope this helps
VM