Yesterday I opened my PC hub (after like, 5 years) to add more RAM and a new HDD. It was full of dust and balls of fluff, so I cleaned it a bit by blowing. I also removed the fan that was attached to the motherboard (I think it's there to cool the processor) and I cleaned it also by blowing and with a paintbrush, then put it back where it was (it had 4 weird plastic screws, it wasn't easy).
Then I added 2 x 2Gib of RAM (Kingston 1333MHz), and the new HDD (Western Digital Caviar Blue 1TB SATA 3). I couldn't find another wire like the one my first disk had, which was thicker, but there was one that was more flat, so I plugged it and it worked fine (I also plugged the wire that I think is for the data, which looks like many little wires glued together).
I'm running Ubuntu 12.04, and in the new disk I've just installed Windows 7.
Between yesterday and today, two reboots ocurred while working on Ubuntu. I haven't used Windows 7 that much so I don't know if it will happen there too.
So where can I begin to debug this?