Why does my Computer get so hot?

Hi

My computer gets hot when i play games.I dont get why this happens.We clean it regulary,piece by piece.In fact lets go back one week.

My uncle got my his own PC cause he was buying a new on.The PC that he gave me looked like it was made in 1986 in Chernobyl Nuclear Power Plant,actually i think that it was made in 2007.So my brother and i took everything out,literally everything and we blew it with a compressor.Before we put everything together we got a new graphics card(AMD Radeon 6670 HD) and then we put it back together.We turned on the computer and it worked well,i bought new windows(7 ofcourse 32-bit) and we installed it.Then we got drivers and what not.We installed a few games(Call Of Duty 4->every setting high and extra,Slender:The Arrival->graphics highest,everything else low,solid gameplay,The Sims 2->highest)but the Computer case was mildly warm/hod.One week passed(today) and we got a new processor Intel Core 2 CPU 6600 2,40 GHz(2 CPUs)-the pc is now medium-high end so we installed Burnout Paradise and it work on highest settings possible but there was one major problem(isnt there always?),the case got fairly warm,even hot( i actually used it to warm my hands)

WHY is it heating so much is it the motherboards fault?

When im browsing the internet the case is cold but when i start a game it gets hot(the games work perfectly).

We cleaned the fans,everything.

The only thing could be that there is no case fan and its about 5 cm(yes im from europe) from a wall,also its on a table not on the ground.

When we first turned it on with onboard video card it was literally cold.

We put thermal paste on CPU but it doesnt work

Im afraid of frying it.

PLEASE help me and sorry for my bad english