People could also install a program called a keylogger on your computer.
While this wont let them link to your computer it will allow them to see everything you type, including text, passwords, credit card details and on on.
What makes you suspect this sort of thing may have happened?
be more specific ... hackers can (could) access just about any part of your network ... they don't need physical access to the machine (although that would make life easier)
but as chuck says ... why are you worth the effort?
A mate of mine was messing with my computer going into all the settings etc., and now they seem to know everything I go into, that's all. thanks for your comments