Odd freeze/restart problem.

I've been having this problem recently, where if I leave my desktop idle for some period of time (I'm not sure how long, I guess a few hours or more), it appears fine, but when I try to launch an application (such as a web browser or other application), the icon stays highlighted, but it doesn't do anything. It sort of freezes. But I can move my mouse around fine. Nothing crashes, just it doesn't do anything. At this point, I have to hold the power button on my desktop (Dell XPS, not sure about the details) until it manually shuts down. I have to press it again, and this is where the annoying part starts. Every time it does that, I get the "start windows normally" thing, and I don't mind that. But after that, it does a "disk check" or something like that, where I assume it checks all my "large discs", er, files for "bad" ones, and it doesn't find any. It gives me the option to press any key to skip it, but even when i press several keys, nothing happens. This "disc check" normally takes up to 5 minutes, and can be very inconvenient if I'm constantly switching between computer and other things. Any help would be appreciated. Thanks. (By the way, I'm only 13, so I don't fully understand a lot of things about.. computers in general, so it's probably a regular problem i can fix easily)