I think that someone might be using my Internet service, what can I do?

[InternetExplorer]

A few days ago I was having a lot of problems connecting to the Internet, so I called my ISP and they said that they would send someone over. They sent someone over and the guy that came over brought a laptop, a cord thingy, and a clipboard with all the info on it. When he came in my room, he saw that my computer had no Internet connection. So then he tried to connect to the Internet by hooking up his laptop to the modem, when he did that he got an Internet connection. And then he did some stuff on his computer like change some settings, and they he said that he would call at 1:00 PM. I waited and about 15 minutes after 1:00 he called, and he asked me if I had turned off or disconnected the router and/or modem, I said yes, and he said that I did not need to do that and he had me turn them back on. After that he told me to log in to my account and check if the Internet was working than, it was working. I think that he might have fixed it while he was at his office or somewhere, because how else could he have fixed my Internet connection without him being in my room with my computer? I think that he also might have gotten acces to my computer and fixed it by changing some things in my computer, you know like you can use remote assistance or desktop assistance in Windows XP/Windows Vista. I think he might have messed with my computer and changed some things, because my history was deleted, and some icons were moved/deleted. I am the only one who has access to my computer. I also think that he might now be able to track what websites I go onto. Like he gets the address of the webpage(s) that I am on right now on his computer and he can see. Now I don't feel safe doing online banking or shopping. I think he might also be able to use my IP address or be able to use my Internet connection wirelessly, I don't want him to see what pages I go on. How can I make sure that no one is watching me or tracking my site visits or using my Internet connection? Is there something I can download to secure my Internet connection from people not using my computer? I am using a desktop computer running Windows Vista Ultimate and my router name is Linksys and my modem name is Zhone.