What is this PC doing!

In the last 2 - 3 weeks my Windows 7 system seems to have markedly slowed down, effectively locking up at times. I do have a system backup I could return to but that would leave me with a lot of updating to do, so that is a last ditch effort. Every so often programs lock up and I notice that there is a lot of disk activity at these times. I can't get into Task Manager to see what is happening as by the time it lets me in, the problem has gone! Even if I leave that running all the time, I can't switch to it as the system is effectively locked up during these episodes. No activity on my modem so its not communicating with anything. I've not installed anything other than security updates for W7 and Kaspersky. It is a heavily used system, with Photoshop and CorelDraw often running at the same time, but this problem is new, but my user habits haven't changed. Windows Defender doesn't report anything amiss, and neither does Kaspersky.
I can post full details of my PC and a Hijackthis log if anyone can offer help. (Is there a Hijack version for W7?).

Uninstall Kapersky and install lightweight free MS Security Essentials which won't slow you down. It will displace Defender. Use Malwarebytes for on-demand scanning.

Study Event Viewer>Admin view to google repeat errors to resolve them. Ditto the Performance log on Advanced Tool page accessed by clicking your WEI score at computer>Properties. Look for cued issues at top of page, also Generate a System Health Report.

Clean and order your HD perfectly with state-of-the-art free CCleaner and Auslogics Disk and Registry defraggers monthly.

Test your RAM with memtest86 CD for 5-6 passes, then test your HD with maker's diag/repair CD scan.