Just going to jump in here with a suggestion. Try using a different power supply-- If you have another computer that has a working psu swap it and see if that helps.
I just went through 2 weeks of trouble shooting a very similar problem. Everything checked out, but had problems getting past the boot screen. On some occasions I got so far as starting windows (XP) before things would crash or lock up. I have a power supply tester (it tested working). Long, long (2 weeks long) story, the short version is: after trying multiple mobos, and a couple of cpus, it ended up being that the fairly new PSU was at fault...literally. Turns out it would power up, then (I believe a heating problem in the psu was the prob)..then basically faulted and could not supply full power to the mobo.
Even if you can find a 450 watt psu to try out, it should be enough to power one harddrive, a vid card and the processor safely into win7 without all of the error messages you are getting.
It really sounds like you are having a hardware problem, and it could either be the mobo or the PSU.
The PSU should be fairly easy to rule out.
good luck
-LP