I'm having the same problem since the upgrade to 2010, I turned off scheduled scanning and just use manual scanning till it's sorted. The auto scan seems to run randomly for some reason.
You might consider contacting Technical Support Chat via the Useful Links at the top of this page. I believe this is a known issue they are working on getting resolved soon.
More than a year on I have almost the identical problem. On my laptop using Vista Home Premium and as up to date as it can possibly be I find that I simply can't set up scheduled scanning. When I try, I click it to "on" and whatever I do next, it instantly switches it back to "off". Naturally it doesn't tell me that it has done so but if I renew the scheduled scan page it is back to "off". If I don't go back to the page, it simply doesn't scan. It would also seem to me to make more sense if finalising the setting and clicking "Apply" were to take me back to the home page but it leaves me dangling and unsure if anything has been recorded.
I had a similar problem with this a month ago on an XP Pro pc where the scheduled scan would only accept my schedule set if I entered the details in a specific sequence, not the physical sequence of the relevant fields. Sadly I didn't write this down at the time but I think it was essential to give the time of day before setting the day(s) of the week!!! Also, it was essential not to use a leading zero for the hour but to use one for the minutes. I didn't actually want to start a scan at 05 minutes after the hour but I checked this by experiment.
What do I do next apart from running manual scans all the time like gpassler?
I have had problems with my scheduled scan starting at times seemingly not related to what it is set at. I have it set for 2:15 AM on Saturday mornings. I have set and reset the scan time and checked my computer clock to make it sure the date and time are correctly set..
Today, Saturday Nov 26th, 2011 at 10:17 I am at the computer. There is no scan running, but when I bring up the McAfee "scheduled scanning" menu it says Scheduled Scan: ON Next scheduled scan Saturday, November 26th, 2011 8:52 AM. Unless there is time travel I do not see how this is possible.
I have the scan scheduled to start at 2:15 AM on a Saturday so that when I normally sit down at my computer on Saturday afternoon or Sunday evening the scan is complete. However what I often find is that on Sunday evening I will sit down at my computer and the scan will appear to have just recently started and it will indicate something like Scan 32% complete and it seems to scan very slowly even though I have it only scanning one hard drive.
This is on XP Media Edition computer McAfee ver 11.0 Build 11.0.623
Scheduled scan starting is controlled by the feature idle timer. It was added as many users did not like the scan starting when they were in the middle of doing something but the feature itself has issue.
It starts the scan when the cpu use has been less than 10% for a full 10 minutes after the set scan time. If this not achieved it waits another 10 minutes and rechecks the usage. If this happens for 2 hrs and no scan starts it (the scan) should be forced to start. The problem is if you shut down the PC before the 2hrs is reached the 2hrs setting resets and the cycle continues.
The next version of Mcafee (2012) should have the option to disable the feature so scan starts on time. If you want a registry work around that forces the starting on time Private message me and I will pass it to you.