Without any information whatsoever I can only generalize a response. Scan taking overlong times to complete can be due to lots of things, CPU being used by other applications causing McAfee to 'throttle back', disk read errors (hard drive needs to be checked using the chkdsk transaction), heavily zipped and re-zipped files, a compressed drive, corrupted files (often the result of using registry cleaners or an infection).
I might be able to assist more if I knew what operating system and service pack you are using and what version of McAfee protection.
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