Ical reads calendar invites inaccurately

I recently bought a new Macbook Pro (10.2.8) and the entire changeover from my old Macbook (10.7.4) has gone smoothly except for iCal, which is behaving bizarrely. When a client sends me a meeting request, double-clicking the .ics attachment opens a random meeting requested by someone else at some time in the past. I've noticed two things:

1. If the sender is also a Mac user, the email comes with its notes including a text summary of the time and meeting topic, so I can right-click on that and add it to my calendar that way - effectively bypassing the .ics file.

2. If the sender is a Windows user, I get nothing but the .ics file. I can see the meeting details accurately using Quick Look -- but the moment I try to add it to my calendar I get bounced back to some random meeting again. I'm reduced to typing the meeting details in manually, reading the information from Quick Look.

This is ludicrous, and it's going to lead to mistakes and get me into trouble.

Can anybody help? Thanks!