My first thought is that they've installed some type of VNC-type software on your pc...like logmein free or uvnc server.
You can restart in safe mode...run msconfig and see if anything is autostarting (startup and services tab), and disable anything sketchy.
If you aren't familiar with msconfig, this may be too advanced a step for you.
The most base thing you can, at least, attempt...is to run add/remove programs (XP) or programs and features (Vista/7) and try to uninstall programs you don't use or that have 'vnc' in the name if you haven't installed them.
Best of luck!