I have set up the account of my youngster so that his activities are logged.
However when I look at the the activy logs (especially log ins and outs):
Click to view attachment
I can only see his activities on and after Nov. 1st 2007.

I therefore suspect that Vista is saving only activities of the current month or for few days.
This is why I am looking for settings by which I can set the duration how long past activities are saved.
I could not find any suitable entries in the registry (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Parental Controls).

Can anyone give me a hint?

Thanks in advance.

Do you have Event Logging enabled on the system? I've read that Parental Controls depends on this service (although it isn't listed as a dependency in the properties of the Parental Control service).

A look at Event Viewer will show you the events that are logged for it.
Go to Start and type in "eventvwr.msc" (without the quotes) and press Enter
Click on the + sign by Applications and Services logs
Then on the + sign by Microsoft
Then on the + sign by Windows
Then on the + sign by Parental Controls
Then click on Operational to view the logged events

Unfortunately, I've setup up Parental Controls on my system, but haven't used them - so I'm not aware of any specifics past this point. You can set the maximum logfile size and retention policy by right clicking on the log (in this case Operational) and selecting Properties.

Thanks a lot for your great tip!

There are lots of details in the event log of parental control.
I found events of the following types:

1 SettingChange (by admin)
2 GameStart
3 UrlVisit
6 Media Playback
10 FileDownload
18 WebOverride (by admin)

2 through 10 are listed only for my son's account.
1 and 18 for the admins account.
You can filter the eventlog by user, event-ID and by date.
Actually the events date back to 2007-10-06 (that was when I set up the computer). Hence the logfile seems to be

big enough.
Very good.

However in parental control there are no events about user login/logout.
So I looked for other categories:
Winlogon
does not contain any events (although it is enabled).
UAC contains only 2 errors.
Seems as if parental control gets login/logout dates from anywhere else.

I'd suspect that there's issues with the logging setup of Parental Controls on your system. Try disabling the Parental Controls - then setting them up again from scratch and see if that helps.