Reminders keep popping up after being dismissed - help!

[fletch032]

We've recently upgraded approx. 80 employees from office 2003 to office 2007 on an XP platform environment.
We have now had our 2nd case of an employee having old outlook reminders that have been set in their calendar popping up after being dismissed repeatedly and I cannot find a solid fix on the 'net anywhere. Basically when you open outlook the reminders will pop up, you dismiss them, close outlook and re-open it and they appear again - very annoying!

Things I've tried so far to fix this problem:
Start>run>outlook.exe /clean reminders - doesn't fix it
Manually opening each reminder in the calendar and closing it - no good either
Opening up the .pst file on a dual-boot XP/vista environment in office 2003 (note - this seemed to fix it for a day and killed the old reminders, but now reminders from yesterday are popping up again after being dismissed!).

With the first instance, it was appointments being sent from one person to another - the opening of the .pst in 2003 seemed to fix this problem.
However, with the 2nd instance, it's just reminders created by the user and not sent to anyone else.
About the only other thing in common is they are both laptop users....

Please can someone shed some light on this rather annoying problem!

Thanks