First off, I am not Microsoft; just a former tech support.
The usual reason for a screensaver not kicking in, is some recent software
change that is interrupting the process.
These can happen at any time on the internet, if you're not extremely
sensitive to them.
Before going through all your software to see which one it is, try this.
Just go to screensaver section of Personalization, change the screensaver,
apply it, and change it back. It's worked for me many times.
From your note, you've probably tried.
Try #2: A computer restart works many times, as well. (stops a local running
process in memory, but not one that loads on startup)
If neither of these works, you could try a restore point, but that's
problematic, if Windows has updated since the restore.
I advise strongly not to restore to before a Windows update.
Keep startup processess to a BARE minimum, and do a disk cleanup, (for temp
files and cookies) and spyware scan.
Good luck!
Ralph