I'm more worried about virus then anything else. Usually Guest account is restricted and virus like Code Red cause that. Why don't you get yourself a software called Pest Patrol from :
http://www.pestpatrol.com/ they do have on line scanner too. After you are done with that, create an account with Admin rights for yourself then see what happens.
Also if you have access to registry you can change the home page in there:
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Main (on the right side of the screen) you can find the reg key for your Home Page, change that to what ever you want.
cheers