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Make sure all open applications are closed, especially your web browser.
Restart your computer and run the setup executable and see if it installs.
Do you have a previous version of Silverlight installed?
If so, try uninstalling the previous version first then try installing again.
Also do a clean boot:
1. Click Start, type msconfig in the Start Search box, and then press ENTER.
User Account Control permission
If you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
2. On the General tab, click Selective Startup.
3. Under Selective Startup, click to clear the Load Startup Items check box.
4. Click the Services tab, click to select the Hide All Microsoft Services check box, and then click Disable All.
5. Click OK.
6. When you are prompted, click Restart.
7. After the computer starts, check whether the problem is resolved.