How can I remove this screw?
Those are external line head screws, and are used as anti-tamper screws in some Japanese electronics.  You can probably find a driver for them from a game console hack supplier (they're used on Gamecube, Wii, and some other game consoles).  The driver looks like this (sorry for the horrible pic):



They also come in 5 and 7 point varieties (hard to tell how many points from the picture you posted).  I've also seen them called torx inverted.

If you don't want to get a specific driver for them, you can probably get them out by clamping on the head really hard with a pair of vise-grips or use a screw extractor.  You might be able to find a socket that will mesh with them well enough, but that's a long shot.