Just replaced the cooling fan in my T40 with a used, but serviceable fan, due to "fan error" message. The T40 is a type 2374, model BU7.   PRIOR to the fan problem, the computer ran fine.  The new fan runs fine, but now the computer won't boot. When I turn the machine on, it starts to boot (the fan spins and all of the green indicator lights at the BOTTOM of the LCD come on), but after it goes through the first three boot-up screens (the” IBM Thinkpad” screen...in color, the "Microsoft Windows XP Professional/Microsoft Windows Recovery Console” screen" and the "Start Windows Normally/Safe Mode/Start Windows With Networking/Safe Mode With Command Prompt" screen, I hear a click and the machine shuts down instantly. The total time from turning the machine on until it self shuts down is 19 seconds. The "click" sound is singular and seems to be coming from the HD. Further, I'm not hearing the normal boot-up beeps. The machine runs perfectly (sans the boot-up beeps) until about 3 seconds after it displays the last start-up screen (the previously mentioned "Start Windows Normally/Safe Mode/Start Windows With Networking/Safe Mode With Command Prompt" screen), then it shuts down instantly with the single click.

The battery is in the machine, it's charged and the A/C power adapter is plugged into both the machine and an good outlet. The battery indicator light (at the bottom of the LCD) is green (with the "+" sign) and remains on continuously both with the machine turned off and during the short, failed boot-up process.

I've also noticed that the green HD indicator light (between the "A" and "Z" indicator lights) flickers during the shortened/failed boot process, leaving me to believe that everything is OK with the HD (just guessing here).

The "1," "A," "Z," and "Cresent Moon" indicator lights at the bottom of the LCD all illuminate instantly when I initially turn the power on, then after no more than 1 second, the "1," "A," and "Cresent Moon" lights go out, leaving the "Z" light on (the "Z" light remains on solid until the computer shuts down...again, total time, 19 seconds).

The HD was previously removed to copy files onto a new computer I got when the fan went out on the T40 (I didn't realize then that all I had to do was replace the fan...real novice here... , but it's back in the HD slot solidly and I've been assured by the professionals who "transferred" the data that it was ONLY copied over to the new machine, not actually transferred. They assured me that everything that was on the HD when I brought it to them is still on it. They further assured me that they NEVER even began to mess with and/or copy any of the software or OS.
I still have the computer "apart" (from installing the fan), but everything's plugged in properly and double checked, including the keyboard and palm rest. The fan seems fine and runs the whole time that the machine is running.

I've removed the HD and tried booting.   The "IBM Thinkpad" page displays for 25 seconds, then I get a page that displays the message, "Media Test Failure, Check Cable," then (as before), the machine shuts down....BUT, this time without the "click"  noise.     Note that the machine runs longer (for about 45 seconds) without the HD before shutting down (in the exact same way as described above).

Have also tried checking the BIOS settings to be sure the machine is reading the HD.   I'm not able to access the BIOS pages while the "IBM Thinkpad" screen is displayed, however the machine does "beep" when I hit the blue "Access IBM" key.  Previously there were no beeps whatsoever.   Anyway, once the second page comes up (the one with the "Media Test Failure" message) I am abble to access the BIOS pages.  However, before I can do anything, the machine shuts down.    Have tried this both with the HD in and out.   Doesn't matter.   

I'm having a hard time BELIEVING that the HD is bad as it was working just fine prior to the fan problem with the machine.   But, then again, I'm a novice at all this.

HELP!!!!!
Did your fan you replaced have a 3rd wire for fan speed tach? If you replace a 3 wire fan with a 2 wire fan (or) a fan that is not the same RPM speed you can run into these problems. Laptops usually have 3 wire fans and when the Tach (fan speed) is not detected they shut down to protect themselves from thermal melt down.