OK, I figured out what my problem was. I had Trend Micro Internet Security 11.20 antivirus installed on my Windows XP system. As a part of the antivirus software, it also included a firewall. Although the firewall was disabled, the drivers for it still loaded. To fix this, I renamed the following registry keys:
After I did that, WinpkFilter worked! Thanks to all that helped!
– Mike B