Generally, if you have a large amount of bitcoins on a given PC, being extra-cautious about third party software (be it an Alt-coin client or a particularly fancy casual game) is advisable.
you should never have a large amount of bitcoins on a PC! better store them in an external disc that was used a safe pc.
It is best to store Bitcoins on cold wallets (hardware). It seems to me, now this is the most reliable and correct decision.