I had hardware update today - moved from Core 2 Duo to Core 2 Quad, but Boinc Client (6.6.36, system: Windows XP SP3) still reports 2 CPUs. Tried running benchmark and it shows 2 CPUs... How to fix this ? Reinstall Boinc (wont there be problems with already processed data) or there is some option (which I couldn't find) I should use ?

What's your setting for "On multiprocessors, use at most x% of the processors" in your preferences?
Are you using web-preferences, or local preferences? If the latter, check this setting there as well.

Does Windows report more cores? Check Task Manager->Performance tab to see how many cores it shows there. If only two as well, it isn't a BOINC problem but a Windows problem. You'll need to reinstall Windows.

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"On multiprocessors..." use at most 100%
local preferences
windows sees more cores - 4, I see it in 'task manager' and in system performance counters (in cpu time counter, I can choose 0-3 processor number)

and this is important - boinc no longer attaches to single core (affinity), now it just uses 50% of total cpu power - all because I can't force it to see all cores

- Open BOINC Manager.
- Advanced view->Advanced->Preferences.
- Click Clear.
- It should now use the web-preferences of the last project where you changed these prefs.

You can then go Advanced-Preferences again and just to be sure change the 100% setting to 100 again. Save them with the OK button. Perhaps that something went corrupt in the actual global_prefs_override.xml file.

Reinstalling BOINC shouldn't matter. These preferences settings are read from and written to files on disk, which stay in place between uninstalling and reinstalling BOINC.

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Problem fixed, this is how I did it:

1. confirmed I had 'on multiprocessor systems use at most 100% of cpus'
confirmed I had 'Use at most 100% CPU time'
2. changed both above values to 50%
3. in log I saw: 'Preferences limit # CPUs to 2'
4. changed above values back to 100%
5. in log I saw: 'Running CPU benchmarks', 'Benchamrk results: Number of CPUs: 4'
6. now Boinc correctly uses all 4 cores

IMHO this is a bug (in newest Windows Boinc version 6.6.36) - after processor change Boinc should 'see' new cores and update values automatically, and even without that - running benchmarks also should fix this... restart boinc also didn't help - I had to use trick I've desribed above