hiwatt.I recently installed Comodo's firewall on my new PC.
I had nothing but problems,for example I could not switch the PC off,was very slow like yours.
One of the reasons for my issues I believe,was down to certain programs requesting access to the net.The pop up request box would lock/freeze.
After an hour of hell,and restoring the PC back to how it was when delivered,I now use the pre-installed Bullguard.
Must mention that I have used Zone Alarm's free firewall,worth a look.
I hope this helps?.