I've just recently been experiencing difficulties on startup. It tends to go like this:
after turning the pc on it freezes before requesting the user password.
The pc has to be turned off at the wall.
When it's turned back on vista runs startup repair, an error is found quoting "unspecified changes to system configuration" as the source.
Vista then starts normally.
It will then work fine for a day or two until the problem resurfaces. I've tried doing system restores but this works only for a day or two also. Service Pack 1 has recently (within the past month or two) been installed.