There's a good, free open-port scanner at
http://www.grc.com (scroll down to "Shields UP")
Unless you are running a server of some kind, it is unlikely you are getting hit with a true DOS, but there may be people (or worms) trying to hack your machine. In particular if you have an FTP service running, the warez cracker types will jump on it and use it to spread pirate software and movies...