No details of your computer, or how it is set up, so anything we say would just be guesses.
What network? What does it consist of, where is it, and how is it connected to each other and the internet?
If there is a wireless connection somewhere, what wireless security do you employ?
What firewalls do you have installed?
Who else has access to this network?
When I checked my history record I can see that there is history listed for people doing things that I was not responsible for. Can someone log on to my network from their computer and have their history show up on my computer?
If not, how would I have all this history showing up on my computer?