Minecraft – Holding a key (wasd and spacebar) presses more then once most of the Time

Check your keystroke repeat rate/repeat delay and turn it down if you have to. I used to run into the same thing back in the day when playing Unreal Tournament and it turned out that the keystroke repeat was kicking in and that was what was keeping me running in the same direction even though I would have different keys held down.

XP & Win7 both: Start > Control Panel > Keyboard

Wikipedia's article on arrow keys covers some of the history of WASD:

The scheme wasn't popularized until competitive play in Quake and subsequently QuakeWorld made clear its advantages over the older arrow key configurations.

Competitive Quake play only popularised the layout though, as it wasn't the first time they were used:

In the same year that Castle Wolfenstein was released, 1981, the game Wizardry used the AWD keys for movement in a 3D dungeon. Both the programmers of Castle Wolfenstein and Wizardry were users of the earlier PLATO system where the game Moria used the AWD keys.

So they were popularised by Quake and the FPSs that followed and made it default, but they'd been around as a control scheme for a while already.