You can use dynamic SQL
I mean first check the version
Then build your SQL statement using a string variable, for example nvarchar(max)
Then execute it by sp_executeSQL
I think following script can work for this task
-- check for sql server version
declare @sql nvarchar(max)
if (select cast(left(cast(serverproperty('productversion') as varchar), 4) as decimal(5, 3))) >= 10
set @sql = N'CREATE unique nonclustered index ix1_table ON [table] (column1, column2)
WHERE column1 is not null and column2 is not null'
ELSE
set @sql = N'CREATE nonclustered index ix1_table ON [table] (column1, column2)'
exec sp_executeSQL @sql
