On boot up, I get a message, "Windows can't find anti-virus software." However, my anti-virus software is installed and running just fine. I've had this software installed for many years but just started getting the error message a few days ago.
On boot up, I get a message, "Windows can't find anti-virus software." However, my anti-virus software is installed and running just fine. I've had this software installed for many years but just started getting the error message a few days ago.

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PlainFred
None are so hopelessly enslaved as those who falsely believe they are free (J. W. Von Goethe)
If Vista can NOT find your AV program, then it is most likely NOT a fully Vista compliant program.
Now running HP Pavilion a6528p, with Win7 64 Bit OS.
Barry, is this message coming from Vista's Windows Security Center? If not there may be some malware blocking its detection by Vista. Try running a scan with your AV and also give MalwareBytes Antimalware a run just to test the waters.
Fred's questions are very relevant, as something may have changed, making your A/V incompatible with Vista.
Check to see if there is a new program update for your AV.
Deadeye81
"We make a living by what we get, we make a life by what we give." Sir Winston Churchill
Barry,
I have had the same thing happen. Once your PC has fully booted up check your Security center and see if it has been recognized and shows turned on there.
If it is then you will find that you may have performed a defrag or a registry clean and now when you boot up Security center is been run before the last parts of your AV is fully booted and Security Center then reports AV not installed. Also note that if this is the case don't worry as your main AV engine will be active as it will be one of the first to be loaded when you start your PC.
If you find the above to be the case then reinstalling your AV may solve your problem. Also if you have performed a System Restore at all then with some AV this problem occurs as the restore point does not change the AV updates and Security Center then does not recognize your AV. Again a reinstall of your AV should fix this.
I suddenly started getting that error last month with Kaspersky Internet Security installed. It was part-way through the license year and nothing had been done to the pc that might have upset it. It cleared itself after a few days before I got chance to investigate properly. (I just did a very quick test to check KIS was still working and used a different standalone scan to check for malware.) My guess was that one of the updates that KIS automatically downloaded didn't play properly with Vista but the company corrected the mistake with a subsequent update a few days later.