After CMOS change, laptop shuts down during setup

[Citizen K]

Last week I suddenly couldn't start up my laptop as usual after hibernating it for a day. Here are the specifics first of all:
IBM Thinkpad T40p
80GB HDD
1GB RAM
1GHZ Intel Pentium N Processor
WIn7 Ultimate
At first I attempted running a recovery (repair), it started well, but never ended. Tried starting up the laptop following the different F8 settings (last known best config, normal settings, debug). I even tried going into system restore via safe mode, but surprisingly there were no restore points to be found. The repair then worked once, and the PC booted up to the desktop level, then I heard a sound like "crr" and the screen froze up. Trying to re-start never worked, the IBM splashscreen never came until I tried changing the CMOS battery. After changing it, the splashscreen now starts, but hangs there for quite a while (20 secs), which is not normal. Then it tells me "checksum error, change date and time". I press F1 to enter the setup (which it does), and whilst I am entering the time, the whole thing simply shuts down. Tried beating it several times now, but it still shuts down. It even shuts down if I do nothing after the checksum announcement. Is the CMOS battery flat already? Or is it something else?
Grateful for any help.

[Computer_Commando]

Have you always run Win7-Ultimate on that?  It doesn't seem like the hardware configuration would support it.
http://support.lenovo.com/en_US/detail.page?LegacyDocID=MIGR-46451
The replacement battery is probably bad, too.
#19 in the diagram
http://support.lenovo.com/en_US/product-and-parts/detail.page?&LegacyDocID=MIGR-46474
Disconnect the CMOS battery & measure the voltage, should be 3.0VDC.

[Citizen K]

I've been running Win7 on it for almost a year now. However I do agree if you are trying to say that I should be using a newer laptop. I actually switched to Win7 because XP was giving me problems. Anyway, checked the battery, but the voltage is ok. I read somewhere else that heat accumulation can also cause this problem (I had BSOD a few times as well) so I decided to check the heat sink and fan assembly. Turns out the thermal paste was kinda old, so I cleaned it up and renewed it. It has not shut down again, and I have been able to successfully run the setup repair. Everything's back to normal. So the shutdown was some kind of overheating protection, I suppose.
I wasn't here long this time, but thanks very much anyway.