Sounds like I had the same, I couln't choose my own homepage either.
What I did was look at the software list in my computer, figured out which programme really shouldn't be there, and tried to delete it.
It won't be deleted, but it will give you a name of a file which cannot be deleted. Let google search for that file, it will bring you to computer-anorak-fora with
detailed files. Apparently there are ways to get rid' of files like that, but it seems only if you're retired or lie in bed with a broken legs (in other words, plenty of time).
It made me give up and reboot my computer completely
What really helps is to get Norton or McAffee, for 25 quid a year I think it's worth it