It may help to rebuild the desktop database -- this is where critical file association information is kept, and in the Classic Mac OS, required "rebuilding" sometimes...
If you've never done this before, it's easy:
1) Restart your computer
2) Immediately hold the shift, option, and command keys all at once and keep them held down as the computer reboots.
3) Keep them held down as the computer goes through all the motions of loading extensions and what-not, and when the Desktop finally appears, a dialogue box should appear asking, "Would you like to rebuild the desktop on (blah balh)?" -- click OK and let it rebuild.
After it's done, restart again without any keys held down to re-enable your extensions.
Holding Shift is optional in that key combination -- command and option held together rebuild the desktop, and shift held by itself disables the extensions during startup. Holding all three does both tasks, and, from experience, I always rebuilt the desktop with extensions off.