First you might try using Syatem Restore to get your system back to a time before this change - in case you don't know how ... go Start > All Programs > Accessories > System Tools > then run through the System Restore calendar for an earlier successful boot time. This might revert back the file association change that occured.
Next read here :
http://64.233.183.104/search?q=cache...LMON.DLL&hl=en
I have linked to Google's cache of the page as it highlights the relevant articles, making it less of a chore to plow through.
Should the errors be the result of spyawre, you might run Spybot or Adaware, update them, then have them scan your pc.
Should it be virus related - turn off System Restore and then have your AV do a scan / Heal - then turn Sys. Restore back on.