Try running sketchup as administrator (or turning your user account
control off)
It may have been working before because you had set the export path to
somewhere outside of program files (the desktop for example). However
if you set the path to the Objects folder, Sketchup is in effect
creating new files in your program files. Windows generally prevents
this to avoid people stuffing up their system files, but running a
program as administrator should get around it. Right click the
sketchup icon and "Run as Administrator" should be the second or third
option down.
If that doesn't help let me know and I'll try to think of something
else