Please let us know which antivirus and firewall product you use....
This is your answer, this is the same website I use when this happens to me.
ON the PC I have that has this happen, it occurs when a Windows Update requires a reboot to finish installing and by coincidence Symantec AntiVirus also downloads an update before the reboot.
The easiest way to fix this is to restore to a prior restore point. Then manually download and install what ever critical Windows Updates you have available. Disable your antivirus before boot, reboot, then update your antivirus.
You need to start at step two...
2. To Boot into Safe Mode and Use the built-in Administrator account
NOTE: You may not need to enable the built-in Administrator account. If this was the only normal administrator account you had, then Safe Mode should automatically boot into the built-in Administrator account.
It’s not complicated, thankfully MS fixed most of the causes of this problem and you shouldn’t see it more than once or twice a year now.
To be more clear, use the link above,
In Step One: Log into Computer, press F8 to get to “Windows Advanced Options Menu”, select Safe Mode with Network Support, press enter to Boot.
THEN:
Go to Option ONE, step 2. And proceed from there.
I have Clam AV for Anti Virus but I use Trinity Rescue for any scans for malware or virus for a firewall I just use Iptables or Firehol. Here is a link to Trinity best little program I ever got hold of!
My Os I use everyday is Slackware 13.1 but I also run NST server which is Linux. I do have a copy of Win XP but no installed, thats why I love these types of threads it helps keep me up to date on Windows.