why is safari opening spam pages?

If you are referring to 'pop up's' you can download a Safari extension to stop this (e.g. AdBlock)

Otherwise, there may be malware on your system. In that case, try re-installing OS X from the Recovery Partition (since there is no way to just re-install Safari)

You might try this:

http://www.thesafemac.com/admedic/

Ciao.

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