Its the same as a house address, if people can find where you live -- they can do what they want -- depending how extensive your security system is.
Finding someones IP is no big deal, and a popup showing it is easy to make, and your browser probably sends the OS you use... I wouldn't worry about it honestly.
Although if you honestly have alot to protect, and don't mind being annoyed and having some system resources used -- download
ZoneAlarm