I can't think of an easy way for 'a' to map to 'return' (sometimes it is easy - 'Ctrl+m' acts as 'return', at least in a DOS shell, so if Ctrl is stuck down 'm' will show this effect - 'j' does as well in Linux) but it might just be worth borrowing a keyboard to check it isn't the keyboard mapping going wierd in Windows. If the new one works, then - as Feline says - just bin it and get a new one.