Add to my previous post
Unfortunately, BSOD occurred again after 5 days of normal run.
So:
I ran HighjackThis again : OK
I ran SmitfraudFix : some suspect entries that were fixed (after Register backup)
I ran CCleaner and fixed incorrect entries
Still erratic BSOD.
BSOD indicated faulty portcls.sys : even if this is not the only possible cause, I disabled Onboard Sound in BIOS yesterday.
So far, the PC run OK.

PC configuration is:
GA-MA790FXT UD5P mb
Phenom II x4 955, no oc
2x1Gb DDR3 12800 OCZ Platinum
HDD WD RE3 250Gb

Next step: I'll update Sound drivers (new version 2009/07/09)
Some useful stuff there:
http://www.maximumpc.com/article/features/blue_screen_survival_guide?page=0%2C0