Overheating CPU

I have a 5-year old XPS 9000.  A couple of years ago, I replaced the heatsink/fan that is mounted over the CPU due to core temps running hot. I replaced it with the same model stock heatsink/fan.  In the past few weeks, I've noticed the computer stalling at times and so checked the CPU temps using CoreTemp and saw that the temps have been running around 70C with little load and ramping up to the 80s when under moderate load.  So I opened it up, took out the heatsink/fan, cleaned it using canned air, cleaned off the old thermal paste (on both the heatsink and the CPU cover), applied new Arctic Silver (grain of rice in the middle of the CPU plate; did not manually spread it), re-installed the heatsink/fan, and then cleaned the other fans in the computer using canned air. 

As sooner as I turned the computer back on, the CPU fan started going really fast and it continued during the entire boot.  Once in Windows 7, I ran CoreTemps, which showed temps in the 90s and eventually 100 and then it shutdown.  Temps were in the 90s again after waiting a few hours.

Is it normal for the CPU to get that hot during boot?  I ask because I am concerned that I may have damaged the thermal detectors in the CPU, which is giving the computer the wrong information, making it think it is hot when it is not.  The fins of the heatsink were not hot, but I didn't expect them to be, given the computer had been on for less than 5 minutes. 

If it is normal for the CPU to get that hot, then I guess that means I haven't reseated the heatsink correctly (although I don't know how that's possible, given the screws are all in the right place and I thought I had screwed them in tight enough).

Is it possible that I damaged the thermal detectors?  I didn't remove the CPU cover, only cleaned it, but maybe some of the Arctic cleaner slipped down the side of the cover?