You might find some help at:
http://www.arduino.cc/en/Guide/Troubleshooting
or
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1187909552/56
I had a similar problem. I've selected the wrong board in the ide. If you have an atmega328, be sure you have selected "Arduino w/ ATMEGA328" not "Arduino Duemilanove".
good luck,
tobias