Run this: select @ @version -- no space between the @ signs.
If it something like this: Microsoft SQL Server 2005 - 9.00.3080.00 (Intel X86) Sep 6 2009 01:43:32 Copyright (c) 1988-2005 Microsoft Corporation Enterprise Edition on Windows NT 5.2 (Build 3790: Service Pack 2), it is 32 bit.
If it is something like this: Microsoft SQL Server 2005 - 9.00.4053.00 (X64) May 26 2009 14:13:01 Copyright (c) 1988-2005 Microsoft Corporation Developer Edition (64-bit) on Windows NT 5.2 (Build 3790: Service Pack 2), it is 64 bit.