I have done a full on -line scan and no virus/malware detected.
I had been using RegscrubXP to tidy up my registry. Seems it might have done a messy job?
For sure, a corrupt of missing registry keys can blow away file associations. Try to do a system restore to a time before you ran the registry cleaner.
If that fails, you are probably stuck with trying to Repair or Re-install Windows.