Freezes all time! Please help..

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My computer hasn't been working very well for the last few days. It all started just a few days ago. I just wanted to do a summer-cleaning in my chasis and opened it. Then I used vacuum to take out some dirt. After making some cleanings very carefully, I re-assembled the chasis and pressed power-on. Then.. nothing.. The fans didn't work, no any sound from the pc but only the power led on. After that, I just went to a computer service to learn what the wrong thing is. They said my motherboard is dead. They sold me a secondhand one for a high price. Then they said my power is also dead. (While their motherboard was working with my pc it suddenly went out and the situation was just like the beginning) and as a result they told me that my power supply is faulty and causes death of motherboards. So, they sold me a power supp and then everything was looking fine. When I got home I found out that my pc freezes alot. Especially whenever I run a game, or WHENEVER I click "Test Direct 3d" in dxdiag it freezes immediately. I tried making a clean install of windows xp but even that was so hard due to lock ups during the installation and even the copying process which takes place first. Now, I wonder what might have gone with my pc... Do you think my video card is also gone because of them? Or is it only their motherboard? It doesn't only lockup when I open a game but also in some other cases... Do you have any suggestion for me? Thank you already.