Reasonable Explanation For Crashes?

For over a year I've been having occasional flurries of crashes, hangs and (rarely) BSODs. Months would go by with no problem, then dozens of crashes or hangs in a week. The event logs and rare dumps have not been helpful. People here said it sounded like hardware problem, but no diagnostics I ran could find a problem.I took it to a pc repair shop which could find no hardware problem (and of course it would not crash or hang for them). The suggested (as did people on this forum) that it probably was ZoneAlarm Internet Security. They installed the Kaspersky Internet Security product (for a fee, of course) and said that should fix the problems. 2 days later it froze 3 times and did automatic reboots 5 times.I took it back. It still wouldn't crash or hang for them. They said Windows probably needed to be reinstalled - that my update from Windows 7 Home to Pro corrupted something. Rather than mess up the installation again I asked them to do it (for a fee, of course).

During their attempts to reinstall Windows it started crashing for them. They succeeded on their 3rd attempt, ran some more tests, checked the configuration, and found problems with the ram. They said ram I had was incompatible with the mother board and buggy, but the problems could not be detected by the diagnostics. They also said they had to reinstall Windows a 4th time because the 3rd reinstallation was corrupted because of the bad ram.

1. If the ram was incompatible with the MB could I have run at all? Could it have caused intermittent problems?

2. I had OCZ DDR3 memory cards and a Gigabyte GA-X58A-UD3R MB. Is this memory incompatible with the MB? I've recently heard that OCZ memory is not very high quality. Is that generally true?

3. Can bad ram (and crashes cause by it) corrupt Windows? I can see how the registry could get corrupted but would that take a reinstallation of Windows to fix?