Acer Says “no Boot Device” On Start Up.?

I have an Acer Aspire 4730Z running on Vista Home Premium. When I try to turn it on it tells me that there is no boot up device. If I hit the menu (F2) as soon as it turns on, I can then exit the menu and log into my computer. I went into the control panel and did a system restore on the computer. I’m not that tech savvy so that’s all I could really think of. This worked and it started fine, I then restarted it, and it again worked fine. When I shut it off, and try to turn it on from the off state, and I got the same no boot device screen. I can’t find the disks that came with the computer. Does anyone have any suggestions?
Also, when I did system repair/restore it said that there was a Windows Update (not sure which one) that was installed a few hours ago. Also, my BitDefender Security 2011 is not connecting, it says that it is turned off, but it won’t allow me to turn it on. The computer works fine when I go from the F2 screen upon bootup, I’m actually using it now.