Find Out Which Program Changed Firefox Settings?

[spacechimp]

My dad has a Lenovo desktop with Windows 8.1 on it, which is only a week old, and running the pre-installed McAfee security suite.

He only browses the web as a "standard user" and he hasn't installed any obviously dodgy software (the only things installed after taking receipt of it are Rapport, which HSBC demands, LibreOffice, and Lenovo Solution Centre, which sort of installed itself) but a couple of days ago whenever he opened a new tab in Firefox it was presenting him with a Bing search window at homepage-web.com/?s=lenovo (which is a dodgy site)

It was easy to change the homepage back but I would like to know what was responsible, and how it was able to modify his settings when he wasn't prompted to enter his password. Is there a trail left when a program modifies a configuration file? I tried uninstalling and re-installing the programs I listed but the problem didn't recur. I'm a bit concerned that whatever changed the new tab homepage could have set itself up to intercept all his web traffic.. Should I re-install Windows to be on the safe side?

(I will run Malware Bytes etc. What I would like to know is how this can have happened on a new computer with antivirus that detected nothing)

Thanks

[Buenchico]

Has your father opened any dodgy emails using that computer. Doing so (particularly if he's then opened any attachments) can result in malware being dumped onto a computer.

[spacechimp]

He says not

[ukbod2015]

I know a lot of people rate Malwarebytes but it seems to come up very often when there are problems on machines.