Moved to VirusScan Enterprise for better assistance. Until someone with that expertise comes along I would add the comment that it is common knowledge to disable virus protection in order to facilitate the installation of Windows service packs.
Usually that error is the result of some corrupted or missing files in Windows 7 and if your request is regarding Service Pack 1 for Windows 7, then....... Please try running the SystemUpdate Readiness Tool from the Microsoft link below and let it repair any items it needs to fix.:
Run the System Update Readiness Tool:
After that, then click on the link below to download the full, standalone installer for Windows 7 Service Pack 1 to your dekstop.. Running the SP1 install from the internet will frequently cause issues.. Download the full installer file to your desktop and burn a copy to a DVD or CD for later use, if you ever need it again. Usually, after clicking on the link below, download the appropriate x86 or x64 file from the bottom of the page is sufficient for most.
After downloading the full installer file to your desktop, then cleanout the Temporary Internet Files folder, plus all Temp folders, make sure the computer is defragged, then restart the machine. After restarting the machine, temporarily disable McAfee, then run the SP1 installer from your desktop.
Hope this helps.