My old iTunes library isn't appearing in iTunes on my new PC
The iTunes library on PC is stored in one of the following locations, depending on the version of Windows in use.

Windows XP: C:\Documents and Settings\username\My Documents\My Music\iTunes\
Windows Vista: C:\Users\username\Music\iTunes\
Windows 7/8: C:\Users\username\My Music\iTunes\

What version of Windows was in use on your old PC? It's also possible that an older version of iTunes was in use on the old PC and iTunes is somehow not recognizing it on the new PC.