USB-Bootable, free, updateable antivirus scanner?

Is there some antivirus that is
1. Bootable from USB (I know there are CD images, which are not preferable at least because they can't save updated signatures even if I install them to USB - which I'm usually too lazy to do anyway)
2. Is free (in my personal case benefit from it is hardly worth the trouble of running, let alone of paying for it).
3. Can update its signatures (and, much preferably, save them so the next update takes less time).

The closest way I can see is installing ClamAV on my bootable linux flash.

But, there are numerous reasons to prefer it exactly that way (rootkit scanning, for example), so I'd be surprised if it wasn't done in easy-to-use way yet.

Norton 360 can, but it's paid !

theog said:

Also
Windows Defender Offline

Oh, thanks, that looks promising.
Additionally, what is its expected detection rate (i.e. chance that it will miss some viruses because of lacking signatures or something)? Esp. compared to other antiviruses.
(quick googling reveals widely varying information...)