After many hours of googling I came over a thread on the Cubase forums.
http://www.cubase.net/phpbb2/viewtop...d679905ae3a5e0
I noticed that the problems issued are resembling mine strongly. I do noticed that every time I get Dropouts I get some Spikes also (from 10% to 100%) and when I get spikes, I hear the Hard Disk grinding and the Hard Disk Activity LED flashing.
The only think that does not resemble is that I am running SATA in [IDE] Mode and not RAID and I use Intel ICH10R not ICH9R as those on the Cubase forum. Firstly, I disconnected my old IDE CDROM and diabled JMICRON in BIOS, but the problem was the same. The sunshine came when I followed Q-Bass's suggestion to set Ableton's process priority in the Task Manager to "Realtime". The spikes, dropouts, glitches or anything else gone away IMEDIATELY . I set the buffer to 48 Samples (2ms latency) and not a single glitch even if I open a game at the same time :P.
Also for those who didn't take a look at the Cubase thread, disabling Multi Core Processing in Ableton's Preferences also fixed the problem but thats not a cure, its more liek a trick. It's like driving a F1 Formula in town with 50km per hour. The Priority thing could be an option but thats also a bypass-kind procedure and I am sure the problem comes from somewhere else.
All that I said above was tested on my old XP x64, but I have recently purchased Vista x64 and I wanted to make use of it. I installed a fresh Vista and with this opportunity I switched to AHCI mode in BIOS (as those on Cubase recomended). This didn't take the problem away though, and Vista doesn't let me to set the priority to realtime, only to High. have no idea why ?
I hope this approach lights an idea for someone, to help me fix this problem once and for all.
Thank you for your time,