If you search BleepingComputer.com, you would find many such cases like yours along with the solutions.
Here is a recent post and the solution was provided by hamluis
One more way to fix basic file associations is to run the RKill
tool developed by Grinler
The tool will not run as exe file associations are corrupt. To work around this, press CTRL-ALT-DEL and open Task Manager. Once there, click File, and click New Task (Run). Browse to RKILL.EXE and click OK.
Edited by Romeo29, 21 March 2010 - 03:40 PM.