i too removed opera 9.7, my machine lockedup each time i would use it.
to set the default browser you have to edit the registry as:
for Opera:
HKLM/SOFTWARE/Microsoft/Shell/Rai/
EFBROWSER
/0 Opera Browser
/1 "\Windows\OperaL.exe"
for IE:
/0 IExplore
/1 :MSPIE