Mine are in c:\windows\inf and they come either from my Windows XP CD or the SP3 install I did. It's 100% Microsoft Windows stuff and they should be present.
Did you clean them accidentally? I learned to not touch anything Microsoft puts on my system, even if I think it might be unnecessary.
Running xp pro, trying to install hp photosmart premium printer. got half way through and stopped - no - certclas.inf, sti.inf, ntprint.inf, usbprint.inf, usb.inf, or usdstor.inf files. Ran system mechanic AFTER the files were not found, and registry is ok. If i can find these files, where do i put them?