overheating help

I built my daughter a PC last year and have never been able to solve the
overheating problems.
It is an Athlon 1400 on a ASUS a7v8x board, 256Mb, CDrw & DVD the case
dimensions are (DxWxH)mm 476x185x443 (cheap case from)
http://www.ebuyer.com/customer/produ...vd19wcm9kdWN0X
292ZXJ2aWV3&product_uid=44509
It has a Thermaltake Silent boost with Arctic Silver 3
An exhaust fan. I cut out the mesh on the back of the case and installed a
wire grill as I felt previously the hot air was bouncing off the mesh and
back into the PC.
I have an 80mm fan at the front / bottom but when I have it running it
increases the CPU temp and decreases the Motherboard temp.
Disconnected it decreases CPU and increases MB temps.
MBM shows the temps with the bottom fan connected sometimes as high as CPU
70, MB 32 not under full load.
With the fan disconnected it shows CPU 62, MB 40 again not under load.
This is all with the side of the case partly off.
The PC is in the corner of the room which is really the only place she can
have it. I don't think that is helping as the hot air has no where to go.
The set up was improved after I added the Silent Boost to it just before
Christmas but gradually it is getting worse. It has now got to the point
where it freezes occasionally.
The fans are all blowing the way they should and the Silent Boost is
positioned correctly.

Can anyone give me some ideas? I thought maybe the cheap case wasn't helping
but am reluctant to shelling out a lot of cash to find it doesn't help. If
it was guaranteed then it would be no problem.

cheers

Steven.