using the SFC did not recognize any problems. nothing missing, no integrity problems and it did not recover any of the missing files.
found the usbscan.sys in C:\Windows\System32\DriverStore\FileRepository\sti.inf_amd64_neutral_9d9a7113099a28a2 and copied it to System32 but the next driver install failed with serscan.sys missing. thats also there, copied to System32, reinstall, failed again.
this time a bunch of other files like hpxxx.inf files missing.
probably a bad/corrupt/incomplete Win7 install. but cant remember anything suspucious during the installation.
decided to re-setup Win7 on the existing installation, checked the previously reported files and all there.
plugged in the printer to the USB port and now all worked.
it seems somehow the Win7 install was messed up. from experience with Win7 installs, messing that up is a feat.
however, finally made it work.