as seen on the picture  
my e4300 cpu idles around 51 c
system temp around 43 idle  
there is a 120 mm fan at the back of the case i  have arctic freezer pro 7 , the  case is aopen h600b  
lst month i changed the cpu fan profile to quiet and then problems started, that was the time i realised the system idled at 55 C . it started to crash  frequently, a friend advised to change termal paste  
last night for the first time i changed termal paste  hopefully problem-free. but now i see it only improved 5 C.  
i read some reviews about arctic freezer. it seems normally the temp must be between 30*40.
i have e4300 running@1800 stock clock.  
i want advice on what may be the problem.  

You just told us your problem and the obvious solution for it, you seem to have a poor airflow if you have only one exhaust 120mm, on top of that your CPU cooler is running on a low/quiet setting, which is causing all your headaches. The solution is simple, get a case with good airflow and/or increase the fan speed on your CPU cooler.

yes  
inside the case a little messy because of cables  
and i have three harddisks  
but i thougt that mast explain maximum 10 C over normal 30C

make sure all the push pins are fully inserted, it quite easy to not notive that some of them of them have 'unlciked'.
 
most cases allow for a second fan to be put somewhere even if it not there at stock, i think that would be a bad idea.
 
the thermal pastes often takes a while to 'set' in
 
also although your temps are a little to high there not going to do much damage.
 
also you could try under volting it a little.
 

thanks for the replies  
i will try to install a front fan i will write the consequences

Cable Management makes a big difference.
 
I went through a similar experience trying to cool my processor more.  Mind you it's an OC Q6600.  But i was unhappy with my idle temps, which at the time where 52C on the hottest core with an ambient room temp of 26-30C.
 
What you should do is try to move around your cables as much as possible so that there's little to no airflow restriction.
 
Example:BAD Notice the mess of cords near the hard drive.  It is blocking airflow from the intake fan.

 
Example:GOOD Notice how all the cords are move around so that there's very little airflow restriction.

 
Also set the Processor fan to the highest setting.  Or at least to the highest noise level you can tolerate.
 
If you don't mind the look, i'd recommend removing the front bezels that cover up the 5.25" drive bays.  This will allow air to more easily get sucked into the case.
 
After i did these things i dropped my idle temp to, on average, 47C.

That will not help, since what you need is more *exhaust* blowing the hot air *out* of the case.  Add a second exhaust fan on the back, or replace the current exhaust fan with a more powerful one.
 

From the review of the aopen h600b (http://www.neoseeker.com/Articles/Hardware/Reviews/aopenh600b/2.html), it will *not* fit a 120mm fan in back.  You likely have an 80mm fan in back.  You definitely need to add a second 80mm fan (preferably high volume) in the 2nd opening back there.

f120 mm silent fan installed at the opening for a smaller fan.  
does that cause problems , it is really silent and effective  

Yes, it's even less effective than having a smaller fan mounted there, so while it may be silent, it's certainly *not* effective.  Take out the 120mm and install two good 80mm fans there.  Try a pair of these: http://www.newegg.com/Product/Prod [...] 6835186121  
They are temperature-controlled, so if your case is cool inside, they slow down to become even more quiet.
 
Unfortunately, your overall problem is that you have the wrong case for your setup.  If you care about cooling and having low noise, you need a case with openings for 120mm fans, as they can move more air at lower rpms, thus also reducing noise.

this is my old p4 case  
after reading answers i checked two cases  
thermaltake mozart tx and lian li g70  
seems a lot of space and 120 mm fans
 
any advice on a case silent for %30 home use %70 htpc  living room computer ?
 

This site and similar places should give you some good ideas: http://www.silentpcreview.com/