Not sure if this is what you have, as you haven't mentioned pop ups, but there's been an onslaught of rogue AV problems w/many of my clients lately (XP Antivirus 2010, Vista Antivirus 2010 and various similar names).
They auto install on your PC with no notice or consent, just by browsing certain sites without high security settings in your browser.
Generally you'll get pop ups stating that you need AV protection, malware has been found on your machine, etc.
I ended up having the issue you have (stemmed from the above mentioned rogueware).
The fix I used was manual (as I couldn't even get my AV updates or even get to the sites), but not too difficult.
1) Browse your PC and find your "regedit.exe" file (usually in C:\Windows)
2) Right click on "regedit.exe", copy, and then paste to your desktop
3) Go to the new "regedit.exe" file on your desktop, and rename it "regedit.COM"
4) You should now be able to douleclick on that file and open Regedit
*ONLY EDIT EXACTLY AS LISTED BELOW. ANY OTHER CHANGES MAY PERMANENTLY DISABLE YOUR SYSTEM!!!*
expand that (click on the + sign) and scroll down to ".exe" and click on it
The bar at the very bottom of the window should show that you're on the location:
The first value for .exe should be the follwing:
If it says anything else under the data column (for example "secfile" instead of "exefile"), you need to change the value back to "exefile"
You can change it by right clicking on "(Default)", selecting "Modify", and then deleting the incorrect data, typing in "exefile" and clicking "ok"
This will renable all of your .exe files.
Once you have done this, close out of regedit and delete regedit.com from the desktop so it can't be altered accidentally.
There will be cleanup that still needs to be done, but unless you're very familiar with the registry, it's best to use software.
The free version of Malwarebytes is always a good start.
Hope this helps :)