When I see.
When I see no boot to safe mode, can't reinstall the OS and such I have found it to not be an OS issue but bad ram or other hardware issues. But there are rootkit (see google) infections that the Vista DVD won't overwrite or remove unless we wipe the drive clean. Often people learn why others write and talk so much about backup at that point.
ONE LAST IDEA. Unplug any USB devices and try one last time.
The very last idea is "not your problem." Above you claim "I bought a new HP a1730n with Vista Home Premium and it seemed to work fine for several weeks" so it is well within warranty. I see you also are not the original poster but do claim "Same Problem".