We've had quite a few reports lately that Avast! Antivirus in particular does interfere significantly with installation of VMware Workstation. Disabling Avast!'s protection is not sufficient to solve the problem... if Avast! is installed on the system, it's generally not possible to install VMware Workstation.
Try completely uninstalling Avast! and then installing VMware Workstation. You might be able to reinstall Avast! after Workstation is installed.
Cheers,
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Darius