Inspiron behaving oddly

System Specs: Inspiron 17R Running Windows 8. 8GB RAM; Installed software: Office 365. Adobe Creative Suite - Just Photoshop at the moment; i-Tunes Player; Rhapsody Music Player; 

The last few days my system has been slowing down dramatically. In addition I've noticed some odd behaviors: dialog boxes don't respond to mouse clicks (I have the touchpad turned off); audio skips - think old type vinyl with scratches; I type faster in Word than system can show characters on monitor screen (I don't type that fast!); Apps are slow to open or don't open at all. 

What I've tried: Ran Malware bites scan (found a few minor things which were removed); ran Bit Defender deep scan (identified nothing); ran defrag (system reported it did not need defrag, but I ran it anyway)  Refreshing system. Restoring to factory settings and reinstalling post factory software. (Only Office 365 and Photoshop are currently installed). Running Dell diagnostics - which reports no errors found. 

So...after all of this I'm left confused and unsure what else to try. I;m suspicious that the issue is software, not hardware, but I don't have enough knowledge to be certain. However, because much of this began following an update to Bit Defender, I'm wondering if that is the culprit. But, here's the issue: I went through the prescribed steps (prescribed by the techs at Bit Defender) to identify and repair any errors in their program.  If you are not familiar with that security software: there are several modules and you disable each module, reboot and then open each module separately in an effort to determine if that module is creating problems. I did all of this - twice. In a state of absolute frustration, I completely disabled Bit Defender, rebooted, went online and found that my surf speed was vastly increased. I also found that most of the issues disappeared. However, when I reported this to the folks at Bit Defender, they claim the issue is most likely a hardware issue. [FYI - if you are not a techie you might find the support at Bit Defender more than somewhat lacking] My question: does this sound like hardware or software? If hardware, what should I be checking? If software, does my theory about Bit Defender hold water? I know this forum is about hardware and Dell issues, but I also know there are some tech savvy folks here who just might be able to help me. If so, I will be very grateful. Thanks for reading all of this and if I've left out some vital info, please let me know. 

Irene