Premier League and SPL clubs have until midnight on Thursday, 31 January to sign players. English Football League and Blue Square Premier (Conference National) clubs have the same deadline for full transfers, but not for loans. Clubs in the English non-league pyramid below the Blue Square Premier are not bound by the transfer window system.
After midnight on 31 January, Premier League and SPL clubs cannot sign any players, on loan or full transfers, until the 2007/08 season is concluded in May. This window will stay open until midnight on 31 August.
Clubs in the Football League can sign players on loan outside of the transfer windows, following a seven-day period after the transfer window closes.
Outside of the transfer windows, any club may still sign players on an emergency basis (such as if they had no goalkeepers available) with the permission of their competition's governing body, ie the Premier League.
Players who are out of contract and do not have a club, can be signed outside of the transfer window if they are unattached free agents when the window closes.
From the BBC site