I came across a lot of posts on various forums that identified 80070057 as an HTTP proxy error. I'm assumming that this is the same as the code with the hex prefix and not a completely different decimal code.
Common wisdom said that the first solution would be to try running proxycfg.exe from the console with admin privileges, running it twice, the second time with a '-d' switch.
Of course I was one of the many who aren't running IIS and did not have proxycfg available. Just in case I did download it though, and ran it to no apparent effect.
The second solution recommended is to do a clean reinstall of Vista, which is about where I am at anyway. By 'clean' it seems that I have to remove the current installation first, presumably to be absolutely sure that the new installation does not pick up any crud from the current installation.
To be sure, I think a format of the HDD is in order. A job for the weekend, I guess.