[Alneyan]

Are you using Windows XP with their drive option thingy activated? I don't recall the name, but you should be able to find out what it is under Properies => Strategies, or some such.

This option is activated by default, and as Windows warns you, it can lead to data corruption. I know my computer had a nasty tendency to crash, leading to loss of data on reboot, and Firefox/Thunderbird were particularly badly hit. Turning the option off helped somewhat, though I cannot swear it reduce such occurrences to zero.