Simple Q&A D&D 3.5 (by RAW) XXVII

[Andezzar]

You can also perform free actions normally in a surprise round, despite that round being short of the normal allotment of actions.

The problem is under the effect of a slow spell you are not short of a move/standard action, you are not allowed to perform any action except a move or a standard action. While you are generally allowed to perform Free Actions alongside other actions, the Slow spell only allows you to perform a Move or a Standard Action. A Free Action is neither a Move nor a Standard Action, so it and any other actions besides the aforementioned two are disallowed while under the effect of a Slow spell.

The surprise round is worded differently (lacking the word only) and even explicitly allows Free Actions:

In initiative order (highest to lowest), combatants who started the battle aware of their opponents each take a standard action during the surprise round. You can also take free actions during the surprise round.