Vista - outlook 7 search problem

Hi all
Dont know if anyone can help me here - this might not even be a Vista issue... but here goes... Vista home premium...Outlook 7.
The search in outlook calendar (in a PST file) has been working fine for me for months - I use calendar, go to instant search, type a word and it lists all instances in the calendar, going back years - fine.
Now suddenly, for no apparent reason, it only displays instances from the last week or so - even though there are evidently loads of instances in the PST calendar going back ages.
Any ideas what has happened?
If it's an outlook config thing that I have inadvertently changed, then feel free to point out my errors, but I really can't see anything that sets it up to only display recent items, and I didn't change anything deliberately!!
Thanks all
Trev