Terrible problem! lcd, hp puter, xphome

I just attempted to install a brand new dell, 18″ lcd monitor on a previously working hewlett packard computer that is almost new and to my surprise and chagrin all I got was black and white words on the screen telling me that my computer was no longer working, possibly because I had installed a new item of hardware or software. It will attempt to boot my xp home edition o.s. but it will not and just recycles incessantly. What can I do to this to make it work. I am desperate, I miss my computer so badly I could just bawl. Am I out there or is this a normal reaction from a computer addict. My computer has the partition thing so that if you lose your O.S. you can get it back by doing a system restore. But guess what, it does not work either. Appreciate any help.