Custom actions that are scheduled as "deferred" execute during the Execute sequence's rollback-protected phase. To give those actions rollback capabilities, you'll need to author separate custom actions that undo the work. These are scheduled as "rollback". Rollback custom actions are scheduled before the action they're meant to revert to in case of an error. The following is an example:
<CustomAction Id="systemChangingCA" Execute="deferred"
Script="vbscript">
msgbox "Imagine this changes the system in some way"
</CustomAction>
<CustomAction Id="myRollbackCA" Execute="rollback"
Script="vbscript">
msgbox "Imagine this undoes the changes"
</CustomAction>
<CustomAction Id="causeError" Execute="deferred"
Script="vbscript">
Err.Raise 507
</CustomAction>We'll schedule these during the Execute sequence:
<InstallExecuteSequence> <Custom Action="myRollbackCA" Before="systemChangingCA" /> <Custom...