What's the complete message? Invalid what?
A computer must have an operating system installed for it to do anything. The operating system is the controlling program that enables other programs to run on the machine. If the machine worked before and no longer does, you have either a hardware problem like a defective hard drive or a virus. You may need to find the original install CD for Win XP Pro, reformat the drive and reinstall Windows.
If the computer has never worked before, you need to buy a copy of an operating system and install it. XP Pro is a Microsoft product and they don't produce free boot systems.
If you know anybody that is knowledgeable about computers that will look at what it's doing that may be more help.
Answered Apr 09, 2011
Edited Apr 09, 2011