back when i was a admin for pc #### this was a very common question indeed.
what cauzed it? ..
modem's (card + install)
Virus Checkers (yep , they wont in all cases quit)
and last but not least windows Updates.
my advice, use AMIGA OS like me or use another Proper os, sure it has downsides (like u need atm 10+ year old HW and we dont have a decent browser, but seriously it makes up for all the crap in the x86 world)