@ Cluberti--- Just as you said: it's AV software that did this. After a whole day's trying and searching, the bad guy finally revealed itself: Zone Alarm Internet Security Suite though all Zone Alarm related services are disabled from starting at boot up, it still changed the page file size to system default by dumping extra needed memory into the page file on the very first reboot after its installation, as any antivirus software always does, and lock the pagefiles size. Most uses would not notice this subtle change. I chose "Diagnostic Boot" and "Safe Mode", both of which made the pagefile size intact. Yet I still didn't know which application changed the size. then ghosted my backup os (without any anyvirus software installed), the pagefile size became again manually controllable thanks again for your help and suggestion. Now I'm trying to solve this problem and further suggestions, if any, will be appreciated