Restart computer.
Refer steps here to delete SC file(s):
Win7/Vista http://support.apple.com/kb/TS2363
WinXP http://support.apple.com/kb/TS1776
Then proceed to repair your QuickTime. START > CONTROL PANEL
WinXP - Add n Remove Programs
Win7/Vista - Programs n Features
highlight QUICKTIME and click CHANGE then REPAIR