My computer started getting hot and shutting down a lot. I noticed that the cooling fan was not always turning as fast as usual so I bought a new fan. I did take the old fan out and clean it and it ran really good for a couple of weeks but suddenly today it just didn't come on. So, I took it and the heatsink out and put the new one in. It turns great and is so quiet. But, after a couple of minutes, the computer just shuts down. It will not stay on. We had to take the memory out to get to the clamps to get the old one off but I know without a doubt that we put them back in just like they were. I have taken them out and reseated them several times but it will boot into Windows and then shut down after just a couple of minutes. Then it won't come back on unless I take the memory out and put it back in. So, it seems like a problem with the memory but I'm sure it's in right. Any thoughts?

Did you use new thermal paste, when you repacled the heatsink? Ifnot, then you will need to do clean the old paste off, and replace it. Use Arctic Silver 5, is it the best.
http://www.arcticsilver.com/instructions.htm has the instrucions.

That was the problem. It's running fine now. Thank you. I'm glad it was something so simple. I love this site. I'm learning so much about computers here!