declare @ZipFileName VarChar(50)
declare @SQLCommand VarChar(400)
SET @ZipFileName = 'MyZipFile.Zip'
SET @SQLCommand =
'exec master..xp_cmdshell ' + '''' + 'C:\MyDir\Winzip\WZUNZIP -yb C:\MyDir\'
+ @ZipFileName + ' C:\MyDir\' + ''''
EXEC (@SQLCommand)