Legal Vista is supplied on dvd with a key to activate it. There are no free legal copies available to download. That's why you can't validate it.
I hope your XP cd is legal and you have a key for it, otherwise you will have the same problem with XP.
You will need to format the drive to install XP, how will depend on whether it's a sata or pata drive, or if it's a brand name pc with a recovery partition.
I have a problem, I have installed the downloaded copy of vista. Previously I have been asked to validate the windows by the OS, didn't has the access the validate it, now I am screwed. Now the problem is when I log on to the OS it only gives me the option to either enter the product ID or goto online and buy the code to enter. Now I would like to format the hard drive and install Win xp. Right now I have Win xp CD.
How do I format the hard drive by pressing F12, or any other good why to do it. When trying to install win xp on top of it, it saying not enough space on a hard drive, so i guess i have to format hard drive first. Please advice, greatly appreciate it.