Laptop problems

[Laura6]

I'm having trouble with a Sony laptop.
When I try to power it up, I hear something powering up but the screen remains black. On rare occasions, it will actually boot up but will freeze up a few minutes later.
Took it to CompUsa for repair. Here is what they did verbatim from invoice:
"Looked at system. It would boot up but lock going into Windows in regular and safe mode. After to trying to boot several times the system finally started doing the no post/no video systems. Removed all power and reset system-same issue. Removed drives and battery-same issue. Removed memory-same issue. Reinstalled memory (in opposite slots as originally configured) and the system booted up into safe mode.
Ran a full scandisk on the C partition-ok. Currently running on D partition. Scandisk on D completed without error. Restarted system and was able to boot to Windows without issue. No video artifacting that we had seen at one point previously."

I brought it home. It worked fine for about an hour (which was an improvement at that point) but then it freezed up like it had before.
I then had the same problems I had before I brought it in for repairs.

Any thoughts?

[BuggZ]

Bad memory could be a possible cause since the laptop worked for a short while after swapping the memory around in the slots. It would help to know what model Sony Laptop you have. Some Sony laptops had a problem with memory retention. The memory sticks would not remain tightly seated in the slots. Try removing the cover over the memory (should be on the bottom of the laptop) and power up the laptop while GENTLY pressing the memory up and toward the slot. If the laptop boots, release the pressure on the memory and see if the laptop freezes or shuts down. I attached a small piece of non-conductive foam padding on the inside of the memory cover to put slight pressure on the memory on one Sony and the laptop has worked fine ever since. If the laptop is still under warranty return it and demand they fix it properly.