There are two possibilities:
- The file associations may be screwed up.
- Malware tried to modify his homepage and failed.
Internet Explorer may think that Yahoo.com (his homepage) is a file instead of a website. Try resetting the browser.
Unfortunately, this is a more likely cause. There's a very easy way to check to see if malware is on the system. When Windows pushes out updates every second Tuesday of the month, they also download new definitions for the Malicious Software Removal Tool. Hold the Windows key and then press "R". In the dialogue box that comes up, type "MRT" and hit return. Run a scan, and if it comes up clean then at least you'll be reassured it's not malware.