I ran into this problem again, but ZoneAlarm was not installed. I did the normal things, but decided to see what updates had been recently installed. I guess the computer had been off for a few weeks and missed Microsoft Update KB951748, as it was the latest update installed. I uninstalled the update and tried IE and FF and both worked again. Reinstalled the update and they stopped working. I ghosted the drive to have a backup and decided to install SP3. It failed as well with "Access denied". I found one reference and gave it a try. After installing a program from the Windows resource kit and running a little script, SP3 installed. I rebooted and gave FF a try. It works and so does IE.
Download SubInACL file from
Microsoft, run into to install.
Copy and paste this into a notepad file and save as reset.cmd
cd /d "%ProgramFiles%\Windows Resource Kits\Tools"
subinacl /subkeyreg HKEY_LOCAL_MACHINE /grant=administrators=f /grant=system=f
subinacl /subkeyreg HKEY_CURRENT_USER /grant=administrators=f /grant=system=f
subinacl /subkeyreg HKEY_CLASSES_ROOT /grant=administrators=f /grant=system=f
subinacl /subdirectories %SystemDrive% /grant=administrators=f /grant=system=f
subinacl /subdirectories %windir%*.* /grant=administrators=f /grant=system=f
Run reset.cmd and after it is finished (be patient), try to reinstall SP3.
I decided to add this, mostly in case someone runs into the SP3 "Access denied" issue.