Windows XP Ghosts.

My Dads XP Pro machine has been doing some funny things recently. Both fixable but dont know how they happened in the first place.

First off the machine has Firefox and Adobe reader and all of a sudden FF started wanting to save pdf files instead of opening them in a pseudo FF/Reader window. Easily fixed just reset FF to its default and hey presto all fixed.

The other is even more curious, the sound scheme had changed from Default to Windows Default, adding two annoying sounds the Menu Click and Menu Pop Up in all windows. Easily fixed just changed the sound scheme back to default. Is there a special key combination that changes the scheme or would using Remote Desktop from Windows 8 change the scheme? It's certainly a puzzle.

Have done Malwarebytes, Antispyware, Spybot 2 and MS AV scans all come up clean. Also looking in FF download history nothing suspicious or task manager is not running anything unusual.

Any suggestions as to how these two things may have happened?

Dad experimenting.

LOL he says not.