Win 7 SP1 fails with 800B0100 error

Tosh L450D - Win 7 Home Premium

All other updates for Windows and Office install without problems.
SP1 fails with 800B0100 error
CheckSUR tool passes with no errors in log.
SFC - no problems found

Potential screwup factors
1) Machine is two years old - but still had the original trial of McAfee installed, which hadn't ever been activated! Ripped that out, ran MCPR, installed MSE. Surprisingly, no significant malware found.
2) no malware scans ever run on the machine - Malwarebytes installed, running full system scan as I write this.

otherwise, it seems like the machine just came out of the box!

Once the malware scan is finished, I intend to try a standalone SP1 install and see how that works.

Any suggestions gratefully received

Thanks, JMH - already covered most of that, and read a good few supposed solutions to the error message, none of which have yet worked.
I'm just going to finish installing all the other Important updates (Office mostly) before I attempt SP1 again from WU.
Then I'll get some sleep before trying to install SP1 from the standalone.

Heh! - I know!
Funny thing is that this is the second machine in two days with an Updates issue - the other one is a 643 error with a .NET update (also with a clean SFC and CheckSur), that I've palmed off on MS WU Support :), on the basis that they need a kicking to get the .NET thing right once and for all.

OK _ a little progress, perhaps.

1) I noticed that a VC++ 2005 install wasn't working either, so tried IE9 and got a 9c47 error
2) tried the WU reset fixit 50123 - no dice
3) tried the other relevant fixit (50202) in normal mode - no dice
4) used 50202 in Aggressive mode - the VC++ update installed
Now downloading the SP1 again - I'll leave that overnight (it's 2a.m. for the second time tonight :) )

Urrgghh!
Download of SP1 via WU bombed out at 63%, and refused to restart after a reboot or the usual Try Agains.

Now installing vias the standalone.
WIll report back with progess.

@Vandalus
Have you run CheckSUR yet? what errors are you getting with any other WU's?
Use Fixit 50123,reboot, then fixit 50202 in normal mode, reboot, and then 50202 again in Aggressive Mode - this will clear your Update History (but not the INstalled Updates listing). See if that helps.