I’m no expert, but here’s my experience:
I have CFP installed on Windows XP and on Windows Vista, both with Defender installed and running, and no problems… EXCEPT, when I upgraded from v2 to v4 of CFP, I immediately observed a severe performance impact on the system, especially when copying file to an external USB drive, to a network drive, or burning to a CD or DVD. The approximate time is 10 times longer than without CFP. I’ve been trying to get help on this issue, but no luck so far. The emails I sent to support have gone unanswered, and the forum post has not been fruitful yet (but I only posted it yesterday, so it’s probably too soon).
I set the Defense+ security level to “disabled”, but it makes no difference. Without CFP installed, throughput to a USB drive in a controlled test environment averaged almost 15 megabytes/second for a 2.5GB transfer of 3000 files of mixed sizes and types (ran the test 10 times). With CFP v3 installed, the average transfer rate is just over 1.5 megabytes/second for identical test (again, 10 tests).
The same approximate ratio is observed for file transfers to the network, and to burn to CD and DVD.
I first noticed this issue because my nightly backups were taking 16 hours to complete, when they always used to complete in about 90 minutes.
I’m really hoping there is some configuration that is not set properly, as version 2 did not exhibit this same behavior. But support seems unresponsive, so far. Perhaps one of the experts can shed some light on this, and provide a fix.
Anyway, sorry to go off-topic with all that. Regarding compatibliity with Windows Defender, I’ve not seen any issues (and yes, I unstalled Defender, AND uninstalled my antivirus - NOD32 - on the XP system during some of my tests to try to determine the nature of the problem - but it always comes down to CFP being the culprit in my case). I did not run any comprehensive tests on the Vista system, because Vista is a little more time consuming to remove and re-install everything.
As far as the firewall portion of CFP v3 goes, I think it’s really good. But I sure wish there was a way to defeat the other stuff in it.