My HP Pavilion XT 936 gave me an error messge, that said my CPU fan was not working, and that it was going to shut off in 5 seconds to stop it from overheating... and it did shut right off. I took it to my techno buddy, who sat it on his desk... and of course, it worked fine... I took it home, and it gave me the same error message and I looked inside... fan was off.

Dumbass me, Replaced the fan... plugged it in WITHOUT my monitor... and NEW fan works perfect... runs with no problem.

The Second I plug my monitor in, I get the old error message and it shuts off in 5 seconds. Why does my computer hate my monitor? HOW DO I FIX THIS??

Sounds like your monitor may have gone bad. The result of which is dragging down one of the computer's power supply lines via the video card and killing the fan or putting the system into an odd state. You might try taking your monitor to your tech buddy and having him try the monitor on another system to see what happens.
I'm guessing that when you hooked up the PC at his desk, you used a different monitor?

My HP Pavillion XT 936 gave me an error messge, that said my CPU fan was not working, and that it was going to shut off in 5 seconds to stop it from overheating...and it did shut right off. I took it to my techno buddy, who sat it on his desk...and of course, it worked fine...I took it home, and it gave me the same error message and I looked inside...fan was off.

Dumbass me, Replaced the fan...plugged it in WITHOUT my monitor...and NEW fan works perfect...runs with no problem.

The Second I plug my monitor in, I get the old error message and it shuts off in 5 seconds. Why does my computer hate my monitor?? HOW DO I FIX THIS???
I have tried a new monitor, and the problem is the same. Leads me to believe the problem is not with the monitor. Thank you for trying.

Ok here goes...
The problem is most likely the power supply.
There is a chance it may be the video card shorting as well but you can test it by testing it in another system. I assume you have no other PCI cards in the system. If you do have a network card or other board you will need to isolate it by temporarily removing it from the system to see if it is not causing the problem. Some cards will drain the available supply and cause failures.

Here is how the system sees power.
First the power supply gets 120VAC from your wall and converts it to 12 and 5 volts DC for the system to use. If you look at the power connector you see several wires. Most are for the voltages the motherboard needs for various circuits. Now the main ones are 5 volts and 12 volts. Your fans (including the CPU fan) run off this 12 volts. There are some that are 5volt fans but if you hook it into the motherboard its usually 12 with a signal pin that tells the system how fast the fan is running. Anyway The CPU takes 12 volts and breaks it up to several other voltages. The 5Volts is used for things like the keyboard and its lights as well as the mouse, usb, and firwire ports. The rest of the motherboard the voltages are split up and distributed throughout. Now the video card slot as well as those other slots have power pins on them. If your video card is faulty and is shorting any of those power pins then the system would detect the lack of power and shut down. This is an internal fail safe thing with the chipset. Now the monitor is a 15 pin connector which has the following pins:
Super Video Graphics Array [SVGA] Bus Pinout and Signal Names (http://www.interfacebus.com/Design_SVGA_PinOuts.html)
That page shows no power at all goes out on those pins.
Now if the monitor is plugged in it will be detected by the video card. The video card has a switching mechanism that if faulty it may cause an increased power drain from the available power supply that the motherboard can give. If your motherboard power drops too low it will turn off or freeze up.
Power supplies go bad and when they do most of the time there isn't a warning.

The main reason I suspect the power supply is because of the switching most video cards do to send a signal to the monitor require a surge of power. Usually its not a problem however it can lead to a sudden power decrease and system failure. The PC needs to maintain power at certain levels and if it decreases down past a certain level there is a fail safe circuit that detects this and will shut down the system to prevent it from getting toasted or other failures.

I hope this aids you in your problem.

Steve C.