I have nothing useful to add regarding a Linux boot disk image to run
malware tools on a Windows box. I always question how recent the tools
are and how well they are geared to finding problems on Windows rather
than Linux.

I think you already gave the best suggestion, install portable
versions of as many Windows AV tools as you can find on a USB stick.
As Marc said, plan to use multiple tools and run each one at least one
more time after it finds no errors, rebooting between each test.....
My old routine was to use anti-virus followed by Ad-Aware and Spybot
Search & Destroy because each one found different things. And the
reboot often brought them back. I'm now using Malware Bytes in
addition to Spybot S&D and MS Security Essentials. For anti-virus I
tend to switch between AVG and whatever else is free. I don't let
Norton near any of my machines.

If you want the last bit of safety, then try building a Windows boot
CD or DVD as Andrew suggested and use it with the USB stick. I may try
BartPE myself.... The boot disk prevents the contaminated system from
executing and the USB stick is easy to update with the latest and
greatest. That is my two cents and worth every penny you paid. :-)

Good luck with whatever you do.

Doug.