I received my HS5 paste today and applied it between the cpu and the heatsink. The temperature dropped to 50 °C in idle (compared to 75°C before) and 75°C when surfing and downloading stuff (compared to 100°C before).
So, apparently the temperatures are correct, the cpu sensor is working, and it was a heat issue.
I can now download stuff and surf the net and post in this forum, all at the same time. I can also play flash animations (in browser) and videos (in media player) with no lag.
So...this thermal paste really works..woohoo!
Plus, they say on their website that it takes between 50 and 200 hours of use before it's at it's full efficiency, so things will even get better
I thank you all for your support, ideas and advice and wish you a great weekend!
p.s. To anyone who reads this: If you have a fast computer (anything over 1GHz) and use it a lot, make sure you have a good heatsink/fan combination and buy Arctic Silver thermal paste to pu in between (instructions on their website). It really helps!