causes of potential cpu oveheating and solutions

[Grouch]

first off I figured I'd post this here so it doesn't get lost in the HO.
A little background recently I suffered a few bsods which I believe I have resolved. with the last bsod I grew concerned about cpu heat as I am currently going through a heatwave my room gets hot during the day with no real way to mitigate the heat I've taken to turning off my pc due to concerns of overheat and to try and reduce the temps in my room during this time.
with my rambling out of the way I will ask these questions to solve my problems and to gather info for others that may have similar issues.
what could be causing a cpu to overheat?
what are normal operating temps?
what solutions are there?
are the solutions cheap(dirt cheap)or free?

Possible Causes:
dusty heat sink
bad thermal past
improper heatsink mounting
bad or inadequate heatsink
overclocked cpu

Solutions:
clean dust
replace thermal paste
check heatsink for proper mounting
replace heatsink
replace/add case fan(s)
underclock cpu

I am personally and currently recommending artic mx 6 thermal paste and enermax brand heatpipe cpu heat sinks and artic brand case fans.
I am currently using all 3 and seeing huge improvements on my cpu temps about 100f degree difference.