Updated just now to latest avast.
After rebooting, my HD started to make a lot of noise sounding like it was being totally trashed, and on my side gadget monitors i noticed that it was being written and read at same rate (mb/s), despite having just restarted the system after avast update, the "perfmon" tool showed it was svchost trashing it, so i rushed to the windows update "nov 2014" hoping it would help, restarted the system but the HD was still being trashed, so i then i came to this forum and now my HD just got back to idle/regular mode, tried to open some programs and all seems responsive, but i gotta admit i feared for the HD for a while. the whole process till it got back to normal took about 10/15 minutes (at least it looked like that, but i was panicking...)
relevant specs:
Windows 7 64 bit
SAMSUNG HD753LJ 1TB
also have an external HD (WD Elements 1078 USB) but it wasn't affected by the issue