I was trying to upgrade my iphone 3gs to 4.0 custom firmware however I received an unknown error 13 and now the phone is stuck in DFU mode.

1) How can I get it out of dfu mode? I've tried holding both power and home buttons down together for 15seconds to no joy.

2) How can rectify the error? When I'm in work tomorrow I can try on another pc running either xp or Vista?