Computer stuck on Microsoft Windows XP Home Edition screen

My computer won't load windows.  I shut it down one day a few weeks ago.  The next day, when I turned it on, I encountered the message screen that starts with

 

We apologize for the inconvenience.  Windows did not start successfully.  This could be due to changes in hardware, softeware etc.

 

I did not install any new hardware or software.

 

I selected the option of Start Windows normally.

 

I get the black Microsoft Windows XP Home Edition screen with the green status bar scrolling.  The computer will then flash black, come back to the Microsoft Windows XP Home Edition screen with the status bar stuck, flash black again then Microsoft Windows XP Home Edition screen with the status bar scrolling but after about a second, the status bar gets stuck and the computer stays on the Microsoft Windows XP Home Edition screen.  The only way to turn off the computer is holding down the power button.

 

I tried booting in safe mode but the computer gets stuck on a screen with a bunch of multidisk commands on it.  The last line on the screen is...

 

multidisk(0)disk(0)rdisk(0)partition(2)\WINDOWs\system32\Drivers\agp440.sys

 

Again, the computer can only be turned off by holding down the power button.

 

I have also tried booting from the reinstallation CD.

I changed the bios so that booting from the CD is first.  I get the screen that says press any key to boot from the CD.  When I do this, I get a blue screen that says at the bottom setup is loading files and it flashes a bunch of files it is loading.  Then it says setup is starting windows but this is where it gets stuck and I have to press the power button to turn off the computer.

 

After a few weeks of not using the computer, I tried turining it on yesterday.  It started and actually loaded Windows XP to my surprise.  I was happy because I needed to back up some files.  I did this, then I turned the computer off.  I tried to turn it back on and once again it could not load Windows.

 

Someone please help...I don't know what else to try.

 

My system is a Dimension 4500 desktop purchased August 13 2005.  This problem occurred 3 days after my warrenty expired.