FAN COMES ON AND RUNS VERY FAST BUT WILL NOT LOAD OPERATING SYSTEM

If the vehicle isn't over heating and operating as it should, I wouldn't think you have a bad sensor. Possibly a bad bearing in the fan motor.

2 causes. the BIOS runs it and is SMART to save system damage.
large PC workstation it is. desktop max.

1: 100% normal; if room or cabana is hot or too hot,
2: PC all vents fans and radiators packed in lint , clean it.
3: too many options the box and BIOS runs fans faster on Purpose

got a raid card there, all mine with one runs fans way faster
due to the card has small heat sinks only for real serves , with high CFM banks of fans, and huge baffling.
no system load told, in the box.
clean all that first
if fails still
take the system down to min system config
the if fans happy now
build it backup see at what point fans go wild.

also vast PC have fan controls in the bios
and some can be set to fast. my tiny dell 9020 has that for me using it in my garage or mp3 play. and never overheats.
or using banks of very hot HDD, 6tb each i set it to fast fans.
or the drives will FAIL, I promise.

learn now to measure or find out the heat load firsts

did you just buy it this thing, or out of the blue fans ran fast
history and PC load (things inside CT0) not told

change the thermostat

The fan is connected to an on board system controller and only comes on when the CPU gets hot .If you are worried you can down load a program called speed fan or find other free temperature monitoring programs to check your computers operating temperature.

I hope this helps
Andrew A

Initially test the monitor in another pc, if display available use the monitor in Compaq presario sr1610nx. Load to BIOS by restarting and tapping F1, if the display available, the computer is ok. restart and tap F8, if the advanced boot option screen comes up, the software is ok.
Restart the computer and tap F10 and proceed with a destructive recovery to reload the operating system on the computer

Not a lot of details but here are some thoughts. The fan should not start as soon as the system boots, so it's doing so indicates that you could have a cooling problem. Try some Arctic Silver compound between the processor and heatsink to ensure proper heat transfer. Also check and ensure the heatsink is not impacted with dust and debris, blow it out with canned compressed air and ensure the cooling fins are getting airflow. Download and install the latest BIOS Release (A17). Run the fan test in Diags and ensure that the fan is turning fast enough. Replace Fan and heatsink if system speed is still slow.

The cooling fan is required to operate when the engine temperature rises above the maximum operating temperature threshold.
After the car is started, it will usually take some time for this temperature to be reached.
Ensure fan runs when fan control temperature sensor switch demands.

the thermo switch determine how high and how long the fan need to run to obtain the proper temperature, it will be a consern if the fan doesn't run.
as far as the upgrade of your machine. do you have enough RAM (2GB minimum) and how is the CPU?(1.6GHz or better) can the CPU handle the operating system. Since you didn't mention that, and by reading what you say, I believe the machine is not able to handle the Vista, that is why the fan is running at high speed and non-stop. The CPU is getting hot, and the power supply is pulling lots power.
Vista loading takes approximately 1-2 minutes, dependent on how many applications you have installed and what you have on the desktop.
roll back to XP from Vista is no possible, the only way is to reformat and reinstall XP.

The fan draws allot of amps (30+) from the alternator which puts a load on the engine, if the throttle valve and idle speed control air-bypass valve are sludged up it will cause a stall because the system can not react fast enough to raise the idle to handle the extra load of the fan, clean the throttle valve and air bypass valve, this should do the trick

this system is opperated by fieldeffect transistors it may be obviouse that 1 or 2 component especially the FET may be sort circuting