On the other hand, the boot-time scanner doesn't support many archives - I'm not sure if UPX is supported. So, the boot-time scanner may not find it.
Klez-H (or its twin Elkern-C) is a file infector... so if it's active, I'd expect more infected files to be found on the disk. Maybe it's just a file that was infected previously, "disinfected" later, but piece of the virus code were left in the file...
In any case, you may also try the avast! Virus Cleaner.