Visitors tab: someone is coming to my visitors tab multiple times, should I be worried?

Those visits are coming primarily from a search engine crawler, which is what goes out and 'spiders' the internet for pages to be searchable. The visitor you had those multiple visits from is actually MSN, Yahoo or Google.

So basically, no need for alarm, a machine is visiting your profile numerous times, not a human.
If you'd like to find out some more information about this, feel free to take a look at this forum thread.