Any and all of the above, except the memory thing. if it ran ok before, it's unlikely to suddenly need more memory in the same config.
SS, if you're comfy navigating round a pc, go find the event viewer & look in there for error logs to see what is causing your crashes. The system will routinely generate warnings and error logs that are inconsequential, so don't fret if there are acres of them but it is important that you know the time your machine last crashed to help identify the trouble.
To do so, click start, then right click my computer, then click on manage. A window will open. Select the event viewer and go from there. Helps if you know dates & times of the crashes so you can match them against what is in the event viewer.
Based on the fact you have rebuilt or had the machine rebuilt, I would personally suspect a duff driver, it's the commonest stop error on windows machines. The obvious quick fix is to update your graphics & sound drivers from the manufacturers website and see how you go.
You can look in device manager which will list all connected devices and their drivers on your machine from the same management window where you find the event viewer - just click on device manager to explore it.
If you're still stuck, you can run windows debugging tools to help find the problem, this can often solve windows crashes in minutes but is a little more involved to run, so see how you go with the above first.