Is that not a windows installer error message?
Try rebooting then uninstalling if you haven't already.
I get this on problem machines just choose yes then it carries on with the install over the top.
we have rebooted and tried to uninstall. Still get the same error. Is there a tool that would remove McAfee completely? Sounds like maybe some corruption in the registry..
for a manual removal.
May also want to look at the windows installer cleanup utility as it sounds like the installer has issues with VScan entries.
I should have clarified...
what I usually do in this case is uninstall the agent using the command line frminst.exe /forceuninstall, copy the basic vse8.7 unaltered software folder (as downloaded from mcafee) locally then run that install over the top, you then end up with the error you've had but it does usually continue with the install when you select yes.
Once this is successfully installed you can then put the agent on again to get your standard settings or uninstall vse depending on what you wanted it gone for in the first place.
I would also check the windows installer is up to date and isnt broken, uninstall something small that uses msi to check (if its XP sp1 or 2 SP3 will usually fix this)