yes i think it is possible that you can be hacked coz you hav other connection and you both share in the dial up, it is just like your in a LAN,try using linux OS it has greater security than micr$oft... and download a cleaner try the demo of moo$oft ,
scan your computer if it contains a trojan, etc..or use the task manager see if there is other program running other than the program that is suppost to be running, ...

.:front2back:. is right if your file and printer sharing is enable he can hacked you in a very simple or basic way throug the use of networking. try to perform a full system scan first on your computer then if some virus\trojan\etc came up , clean you computer by using downloadable anti virus...hope this piece of info might help you ..