If you’re running SQL Server 2005 Express Edition on the x64 platform, you'll definitely want to check out the Microsoft download for SQL Server Management Studio Express Service Pack 2 (SSMSE SP2). Earlier versions of SSMSE wouldn't run on the x64 platform even though SQL Server Express ran as a 32-bit application using Windows x64 Windows on Windows (WOW) support. SSMSE SP2 will run on Windows x64 as long as the 64-bit version of the .NET Framework 2.0 is available. You can download SSMSE SP2 at:
http://www.microsoft.com/downloads/details.aspx?FamilyID=ccbda432-8ecf-4c82-bdef-b575eaf07f55&DisplayLang=en