Shutdown during Windows startup

Every morning it takes me several attempts to get Windows 7 up and running. The computer goes through the POST OK and during the "windows is starting" logo (after about 15 seconds) everything turns off (except system fans). No blue screen - just nothing. I have tried safe mode and other modes and have also disconnected hardware (usb, ram, hd etc) but it still happens. Booting from the Windows DVD no problems. After half a dozen attempts the system boots up fine and runs all day, even rebooting OK if I need to. Almost like a car that doesn't like cold starts. Logs only show typical boot up errors. Diagnostics and windows repair can't find anything wrong. Only solution so far is to leave the PC on 24/7 - not good. Everything is backed up just in case one day I cannot start up at all. System is a few yrs old (AMD 64, FOXCONN mb, IDE HDs, 2GB (max) ram) and I had several hardware problems during installation. Any ideas please.

Good question. It has only happened as described since I installed Windows 7 about a month ago. BUT... one reason for upgrading to Win7 was that my previous OS (XP Pro) crashed when I inserted a SD card into the 3.5 inch USB card reader. I was unable to recover from that crash without reinstalling so I went out and bought Windows 7. After that, the card reader couldn't be seen and I had all sorts of problems that I traced to the onboard USB2 connector. I installed a PCI USB adaptor card and now the card reader works OK. So I know I have a mb with at least one problem (I don't know how to disable the unserviceable USB connector). As I said in my original post, all works OK once it has started. I have a feeling that a new mb might be the answer but am trying to exhaust all other options as that would mean replacing everything including the OS as I stupipdly bought the OEM version.