Installed fonts on my Windows PC are not carrying across and showing in PS6

Are you referring to Photoshop 6 or Photoshop CS6 (actually Photoshop 13!)?

Assuming you really do mean Photoshop 6 and you are on Windows, you should be aware that Photoshop 6 was released in September 2000 and designed to work on Windows'98 and Windows 2000, 32-bit systems. Since Photoshop as well as other Adobe graphic arts applications do their own font enumeration and management in coordination with what is built into Windows. Modern versions of Windows (i.e., Windows 7 and 8.x) and especially 64-bit versions of same have changed dramatically from the OS versions for which Photoshop 6 was designed and incompatibilities such as you describe are to be expected.

And if you mean Photoshop 6 and MacOS, there isn't any Macintosh (and MacOS version) sold in at least the last half dozen years or so that can even execute Photoshop 6!

If you mean Photoshop 13 (=CS6), then the question would be what format are the fonts (TrueType, OpenType, Type 1) and how you installed them as well as whether the Adobe font list/cache files need to be regenerated. Those files are the files of the form AdobeFnt##.lst where ## is a two digit number. Exit Adobe applications and delete any and all such files on your system. Then try running Photoshop again and see whether the fonts are found.

Something else you should look at is whether the fonts are in your font list, but listed at the bottom of the list. Fonts that have unusual encodings and or international character sets are typically listed in their own alphabetical list after the Western Roman fonts at the top of the list.

        - Dov