Many people are having problems with SharePoint and Outlook 2007 Alerts. Although there are a number of ways around it you can try the the hotfix from Microsoft; however in my case that didn't work. You can always click the REPLY TO button and you will see the alert, but that kind of defeats the purpose of viewing the alert in the Preview Pane.
Well I have come up with a solution, instead of using Alerts, I use RSS Feeds instead. Since Outlook 2007 allows for viewing of RSS Feeds I just subscribed to the feed and viola, I can now see the entire message anytime I like; and I can search in Outlook 2007 for past articles, announcements, calendar items, etc…
Below is the link to Joel Oleson's Blog, it contains the link to the Microsoft Hotfix, I would recommend trying it, it may work for you; however if it doesn't then I can guarantee my fix will work.