The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
2. Sort and Limit Before Joining (modified query below): In cases where the joins aren't filtering any rows, it's possible to sort and limit the amount of rows using a subquery in the FROM clause, before applying the joins to all other tables.

Optimal indexes for this query:

ALTER TABLE `adstable` ADD INDEX `adstable_idx_adacti_approv_target_target_adbudg` (`adactive`,`approved`,`targetdesktop`,`targetwindows`,`adbudget`);
ALTER TABLE `adstable` ADD INDEX `adstable_idx_imp_today` (`imp_today`);
ALTER TABLE `userstable` ADD INDEX `userstable_idx_balance` (`balance`);

The optimized query:

SELECT
        adstable_adid 
    FROM
        (SELECT
            adstable.adid AS adstable_adid,
            adstable.user AS adstable_user 
        FROM
            adstable 
        WHERE
            adstable.adactive = 1 
            AND adstable.approved = 1 
            AND adstable.targetdesktop = 1 
            AND adstable.targetwindows = 1 
            AND adstable.adbudget > 0.02 
        ORDER BY
            adstable.imp_today ASC LIMIT 1) AS adstable 
    INNER JOIN
        userstable 
            ON (
                adstable.adstable_user = userstable.id
            ) 
    WHERE
        1 = 1 
        AND 1 = 1 
        AND 1 = 1 
        AND 1 = 1 
        AND 1 = 1 
        AND userstable.balance > 0.02 LIMIT 1