you may be able to use it as a main monitor, but you'd have to at least still have a video card in the PC in order for it to boot. you'd more than likely have to boot up with a regular monitor attached, install the usb monitor, set it as teh primary display, then disable the other one and disconnect the monitor. as long as the video card is still in the PC, there should be no booting problems, and once windows loads, that monitor should activate. although because it is USB, i cannot verify that.
also, due to the fact that it is a low resolution usb monitor, i'd suspect that the response time is probably not that good...so for doing slow moving stuff, or desktop icons, it would probably be fine..i have a feelign it would probably suck to actually play games on, there would probably be a good bit of motion blur