Yes there are many variables . For example ; virus , spyware , malware , trojan kind of problems could make problems too.
I doubt the others (spyware ...) but if a virus changed some files it is possible .
I hate this kind of position .
Don't t misunderstand me Wetbanana , i would love to be near your pc and look for the problem . But from internet it isn't so easy.
Sometimes it is a very basic problem . Of course if you know or can see it