Yet another J solution. This one has two functions that mutually recursively build the spiral. This one too accepts the size of the spiral as argument.

spiralh =: (,:@i. , ] + [: |:@|. spirals) ` ((1 0$0)"_) @. (<:&0)
spirals =: ,:@i. , ] + [: |:@|. [: spiralh <:
spiral =: ([: >: spirals)"0
2!:55[0[ 1!:2&2 spiral 5".>{: ARGV
[download]

This is getting addictive, isn't it?