Have you tried dynamic SQL?

Something like this:

declare @sql nvarchar(max),

@Startdate nvarchar(10),

@Enddate nvarchar(10);

set @sql = 'select * from #table1';

set @Startdate = '01/01/1900';

set @Enddate = '01/01/1900';

if @Startdate is not null and @Startdate <> ''

and @Enddate is not null and @Enddate <> ''

set @sql = @sql + ' where Startdate >= ''' + @Startdate + ''' and Enddate <= ''' +@Enddate + ''''

print @sql

exec sp_executesql @sql