A big part of keeping your computer safe online is blocking all the malicious noise on the Internet. To check how susceptible your computer is to being detected by other machines on the Internet, read the information at GRC's ShieldsUP! site, and run the tests. The one I use most frequently is the All Service Ports scan, to make sure that a computer appears "stealth"ed on the first 1056 ports.
GRC, Gibson Research Corporation, is named for its founder, Steve Gibson. Along with Leo Laporte, Steve does the Security Now podcast on the TWiT Netcast Network. You can find transcripts of Security Now episodes on the GRC Security Now page.