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Update! Well I purchased 2 x 4Gb Patriot RAM sticks PC3 10600...and my macbook pro started crashing randomly (after having NEVER once crashed previously).

The crashing got so bad, and also frequent hyroglyphics and random coloured characters on screen. I could only have been bad RAM. I tried a fresh install thinking it might be a virus too. The RAM installed would not allow me to even install the Snow leopard again from disc. Just kept getting errors.

So I replaced the old RAM and it allowed me to reinstall Snow leopard no problem. Clearly the RAM was to blame.

The shop have exchanged this RAM for Hynix RAM 2 x 4Gb sticks. So far installed and OK so will report back if it starts crashing.

Al
 
I tried a fresh install thinking it might be a virus too.
For future reference in troubleshooting, you don't have to worry about a virus, since there are none. Macs are not immune to malware, but no true viruses exist in the wild that can run on Mac OS X, and there never have been any since it was released 10 years ago. The only malware in the wild that can affect Mac OS X is a handful of trojans, which can be easily avoided with some basic education, common sense and care in what software you install. Also, Mac OS X Snow Leopard and Lion have anti-malware protection built in, further reducing the need for 3rd party antivirus apps.
 
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