I've been having this issue with my computer blue screening over the past few months in which I'd turn my computer on and it'd blue screen within about fifteen minutes, but once I restarted, it'd be fine. I narrowed it down to my wireless adapter and someone told me to switch around which USB ports my devices were hooked up to on my computer. So, here's my issue: when I unplug my M-Audio C600 and plug it back in, my computer recognizes the interface..... BUT.... when I open Sonar X1, I get a message saying that, "there are no audio devices connected." So, I open Sonar, go to preferences.... no interface shows up for playback or recording.... even though in CONTROL PANEL the interface is listed as the default device and the onboard Realtek card is disabled. At this point, I have two options: switch from ASIO to WDM (this works) OR reinstall the drivers, which makes ASIO drivers work again. Anyone know why this happens and how to fix it? I should be able to unplug my interface and not have to worry about losing major functionality.....

Anyone have any ideas at all?

Did you download and install the latest driver from M-Audio?
Is your Windows 7 32-it at SP1 level? 
Make sure the C600 is plugged into the computer and not a USB hub. 
Make sure you have the power plug connected to the wall. USB does not provide enough power on its own. 

When you installed the C600, did you follow the instruction and not have the USB cable connected until the installation asked for you to plug it in. That can cause weird problems. If you didn't follow them, uninstall it, unplug the C600, reboot, and re-install the drivers. When the install completes, reboot the computer and then plug in the USB cable. 

Thanks for the reply, Dave.....to answer your questions: - yes, I have W7 SP 1 installed - no USB hub -- going directly into computer - followed the instructions carefully So, that leaves the power cord for the C600. I do NOT have it plugged into the wall because I thought the USB powered it enough to use it like that. The manual says that the power cord simply enables inputs 3 and 4.... it doesn't say that, if you power it by USB, its functionality is compromised. But, I will definitely try that!