Rebooting your computer is the most common first-step remedy for most PC ailments. Rebooting returns all of your computer's software (and hardware) to a known initial state, which in theory should eliminate fluctuating problems. The issue could be as simple as a software error or as complex as a motherboard or hard drive failure, but you really can't narrow down the options until you try a few things. Since you have to take a logical scientific approach to identify the true source of your computer's problems, you should really eliminate all "unknowns" that could possibly exist when trying to get your computer back to a previous "known" state. For example, there could be some hardware that is "stuck" that causes a Blue Screen of Death, and it won't fix itself until the card is powered down (i.e. a hard reboot)... all the soft reboots in the world will not fix the problem.