You can try turning off UAC and see if that helps, but I don't recommend it. UAC does serve a purpose. To disable UAC for testing open up a run box (start menu >>> run) and type Msconfig. This will open up the ms configuration tool. Go to the tools tab at the top and click it. This will show a list of available tools. At the bottom of the list are the scripts to disable and enable UAC.
If disabling UAC fixes the problem then you either have to keep UAC disabled or install AC to another directory. I would recommend installing AC to another directory instead of disabling UAC.
If that doesn't solve the problem then you are probably missing files that are needed for AC. Like directx ver 9.c. Also, it could be your firewall or anti-virus blocking AC, but that is less likely in my opinion in this case.
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Never! Never! Never! take a sleeping pill and a laxative at the same time.