If the machine is off it's not a problem while it's off. But if you are online for any lenth of time you'll get hit - just a matter of time, days, weeks, or a month.
Most everyone on cable systems belong to ranges of IP addresses that are easy to figure out... Some machine(s) that have already been compromised will seek out new unprotected computers, slip a little dormant worm into it and activate it at a later date. Not sure how true this is but (something I saw on the net
) the funny thing is that they tend to look for home users as they are more likely to get in and less likely to be found.
I've changed firewall systems at home but my last one last year had numerous attempts every day in my logs. The last place I worked had numerous attempts too but no known intrusions... Although since I left, they may or may not know if an intrusion has taken place as nobody knows the system