it would be extremely helpful if you would answer the questions those here trying to help are asking, like post # 3 for example. I will try again,
assuming you applied/re-applied the thermal compound correctly.....
1) When did it start spiking temps?
2) Have you recently moved the machine?
3) Have you made any changes to the machine recently?
4) What are you getting these temp readings from?
a) If you are using speedfan for temp readings, try something else. this is excellent
http://www.cpuid.com/hwmonitor.php
5) What temp does your bios indicate?
6) What is the core voltage ?
7) Are you sure that the Heat sink is seated correctly
8) Have you made sure the cpu fan is running,and at the correct RPM, and not slowing down or stopping?
9) Are you sure the CPU is getting that hot?
a) Is it acting strangely? Lagging,BSOD,Hanging up, restarting, etc?
10) Have you check the Motherboard for cracks around the CPU socket?
11) Have you checked the screws/push pins to make sure they are not broken/stripped and applying even pressure on all sides?
what im attempting to ferret out is that this is an older machine and unless it has been running at 100C the whole time...something has changed, what is that something?
If you applied the Thermal compound correctly. it sounds as if the heat sink is not making proper contact, the fan is not spinning at all,or at the correct speed,or you have a bad sensor giving you a faulty reading.
as I see no remarks on your machine performing strangely. you don't list the exact model of your Pentium, but if its a P4 9xx series, top thermal spec is around 64c. If it was running at 96c, you would be having problems like the ones previously mentioned, and should be shutting itself down via the thermal shutdown limit set in the bios (unless it has been disabled) . you can also try the old fashioned method of a temperature check. Open the case and touch a metal part of the chassis to discharge any static electricity, then touch the heat sink. if its 90c+ , you will know it and not be able to touch it for long.
here is a tutorial on thermal compound application.
http://www.svc.com/article-thermal-compound.html