I was using my computer and it rebooted (I didn't reboot it) and when it came back on, I got this message: "Invalid system disk, replace the disk and then press any key." I have a lot of data on the disk and do not want to lose it. It is a SATA hard drive 500 GB and I am using Windows 7 Ultimate as my operating system. One other thing, prior to my system crashing, Windows 7 had been doing an update in the background as I was working on a Word file and printing on some DVD's with my printer. any help or advice would be appreciated.