Stalking

stalking is defined as "the willful, malicious and repeated following and harassing of another person".
...so when does a slight fixation with someone become stalking?

There are several factors to when stalking begins. For example, when a relationship has broken up or turned bad, when the stalker perceives some wrongdoing to themselves, or when the stalker believes themselves 'in love' with the victim. They begin to follow the individual, and it becomes stalking when the victim begins to feel harrassed and victimised.

With the advent of the internet, and social networking sites such as Facebook and Myspace, and instant messaging systems such as MSN Messenger, there has been an entirely new form of stalking begun, called 'cyber-stalking'. The stalker can easily harrass the victim anonymously online over these networks, without being tracked down, which is why internet security is being tightened.

see here for information on stalking, and worryingly enough, how to check if you're a stalker.