I had to reinstall windows and now can use only 2 cores instead of 8 (as shown in Task Manager, CPU-Z and AMD Overdrive). Device Manager shows 8 cores under 'Processors' tab though. No hardware has been changed and the same Windows is used (XP64 Professional SP1). I tried everything I found on internet:
1. msconfig -> boot -> advanced: can't select more than 2
2. reinstalled windows (to other XP64 edition)
3. Device Manager -> Computer says ACPI Mltiprocessor x64-based PC and I can't update it since it says: "The wizard could not find a better match..."
4. reset CMOS
5. updated BIOS to the most recent version (1.70 from 18/10/2012)
1. use single one 80gb SSD) and had to resolve it asap to continue work because of deadline. So it was much quicker to reinstall whole system and set up a new temporary database than trying to fix the problem and fail to do that eventually. I simply couldn't risk I waste 2 hours and still would have to reinstall it after all...
2. It was all OK, except there're some whole system lags in last hour or so which I believed where related to database problem (they were for sure, at least partially).
3. Yes it did for about 2 months since I built it.
Based on your answers, the most likely problem was a corrupt Windows install.
I don't know if you did a repair, a restore, a reinstall over the top, or what the case is, but it sounds like Windows needs to be blown away completely and reinstalled.
What I would suggest is that you put in the Windows CD, make sure to delete all the partitions, and then install again fresh with no pre-existing files remaining.
Even if that sounds like what you did already, I would do it again.
I still don't understand what the original problem was, but you might additionally want to run some hardware tests when you get some free time. MemTest86+ can help with RAM, AIDA64 or Merseinne Prime can help with the CPU, and Windows itself should be able to check for hard drive errors.
- Edit - It did turn out to be a Windows install problem it seems. Glad it is sorted out, at least.