I'm on Vista.
For last several days, at a startup, I've been getting annoying Windows warning:
An unauthorized change has been made to windows
After that, I wasn't able to open Control Panel, get Windows updates, etc.
Validating Windows at Micro$oft site, fails.
I tried System Restore, which to make me more confused, helped for a day, or so, but next time, I start Windows, message is back.
It was driving me nuts.
Search for solution, pointed me to some Vista incompatible program.
The thing was, I really didn't install anything recently, and if even so, System Restore should have helped.
In SAID period of time, all I installed were some Windows updates, so again, System Restore should have helped.
Nothing, no solution.
Finally, I found some POST about PC TOOLS Spyware Doctor, which comes as optional install with PC Tools firewall, may be incompatible.
But, I didn't install Spyware Doctor.
Finally, I've decided to unistall PC Tools firewall (which I've been using WITHOUT a problem for last 3 months), and voila!, problem goes away, Windows validates, no more warning pop-ups.
What happened, I'm not exactly sure. Either some Windows update, all of sudden decided, that PC Tools firewall is non-compatible with Vista (formally, it IS), or PC Tools firewall ran some kind of update (which I didn't notice), and made itself non-compatible.

This is just as a warning to anyone, who could experience similar problem.
All is good now, here. I installed Comodo.