Seems as though your PC doesnt know the default browser to use.
On Windows 7 - Control Panel - default Programs - Set Default Programs - Click on Internet Explorer - Set this program as default.
On XP - From Control Panel, open Add/Remove Programs applet, and choose Set Program Access and Defaults button. [ Alternately, click Start, Run and type control appwiz.cpl,,3 ]
Click the Custom button once. From the list that appears, select Internet Explorer and then click the OK button.
Have you recently installed some new progs by chance???