Did this just happen recently? Was it working before? Has anything changed on your computer like new hardware or software installed or updates applied?
Check for corrupt system files and then try a restore:
To run the System File Checker tool, follow these steps:
Click Start, and then type cmd in the Start Search box.
Right-click cmd in the Programs list, and then click Run as administrator.
If you are prompted for an administrator password or confirmation, type your password or click Continue
At the command prompt, type the following line, and then press ENTER:
When the scan is complete, test to see whether the issue that you are experiencing is resolved.
To restore the operating system to an earlier point in time, follow these steps:
Click Start, type system restore in the Start Search box, and then click System Restore in the Programs list.
If you are prompted for an administrator password or confirmation, type your password or click Continue.
In the System Restore dialog box, click Choose a different restore point, and then click Next.
In the list of restore points, click a restore point that was created before you began to experience the issue, and then click Next.
The computer restarts, and the system files and settings are returned to the state that they were in at the time that the restore point was created.
If you are still getting access denied, try temporarily turning off user account control:
1. Open User Accounts by clicking the Start button , clicking Control Panel, clicking User Accounts and Family Safety (or clicking User Accounts, if you are connected to a network domain), and then clicking User Accounts.
2. Click Turn User Account Control on or off. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
3. Select the Use User Account Control (UAC) to help protect your computer check box to turn on UAC, or clear the check box to turn off UAC, and then click OK.