Go here and download this version of Ubuntu LTS. Be sure to burn the .iso file to a cd-rom to make it bootable.
This is a direct link to the .iso file (694 MB) here: http://releases.ubuntu.com/lucid/ubuntu-10.04.4-desktop-i386.iso
If this is not the file you need then the general download section here: http://releases.ubuntu.com/lucid/
Why an older version of Ubuntu? Version 12.04.1 requires a steep learning curve to use for those used to XP operation.
Why use Ubuntu as a Live CD? No changes are made to the XP system when run as a live cd bootable disk. XP will never know Ubuntu was there as long as care is taken to not install it.
You need to copy and paste important files off the drive and save them before we can go further in helping you.
An important clue will pop up if Ubuntu tries to start, but produces an error. This would likely mean a hardware fault (other than the hard drive) such as memory stick or other. Included in the live cd is a program, MemTest, that can be run and check for memory errors. If the hard drive is not readable by Ubuntu, that would be another clue.
Merely copying the .iso file to cd will not produce a bootable cd disk. This file must be opened by a burner program such as http://www.imgburn.com/ to be opened and become bootable.
Once the files are copied off the system drive, then any work needed to get the system running again will not result in the loss of important files.
Important: Ubuntu can tell you what hardware is installed on the system such as processor type, speed, manufacturer of hard drive, size of drive, model of drive, etc. Once we know what we have, we can proceed without guessing or damaging anything more than already has been done.
An option to get this system up and running again is called a system repair. This is different from a new or clean install in that non-windows files are left intact with the exception of need to do the entire Windows Updates all over again. If going this way, back up first, then repair, then make sure SP3 is done.
There is the possibility of malware in the system; hence the failure to completely boot. In that case, a clean install is recommended vs. a repair install.
Do let us know what the results of running Ubuntu as a Live CD are. Were you able to save important files?