some useful links

http://www.trojanscan.com/ a free online trojan scanner
http://www.iana.org/ a good source for technical papers and port references
http://www.arin.net/whois/ ARIN's whois lookup, can lookup by IP or Domain
http://www.geektools.com/whois.php Geektools whois, by IP only
http://www.robertgraham.com/pubs/ good general information about Firewalls
http://www.dshield.org/ A agency that you can suybmot your logs to, and they will use yours with others to try and clean up some of the bad signals flying around.
http://www.mynetwatchman.com/ Another reporting agency
http://www.accs-net.com/hosts/ Hosts file information
http://accs-net.com/hostess/ Hosts file manager
http://www.memtest86.com/ Geeky memory tester
http://www.safer-networking.org/ Spybot S&D Malware scanner
http://lavasoft.element5.com/ AdAware malware scanner
http://www.webroot.com/ Spy Sweeper, another malware scanner
http://www.winpatrol.com/ WinPatrol
http://www.wilders.org/index.htm Comprehensive Security site