Check you hard drive for errors. Go to Start/Run and enter cmd then click the Enter key.
Type in chkdsk /r (with the space) and hit the Enter key.
It will most likely refuse to run stating volume in use, try at reboot Y/N? type Y and hit the Enter key then exit and reboot.
In Vista do it in an elevated Command Prompt window. Instructions here: http://www.mydigitallife.info/2007/0...windows-vista/
Sometimes VirusScan will appear to freeze, especially at zipped/compressed files as it has to unpack each one before scanning can continue.
Failing that contact Technical Support chat, available 24/7 via the link at top left of this page or in my signature.