This assumes that your missing emails were in your Inbox. If they were elsewhere (such as a folder you've called 'Friends', modify the following instructions accordingly.
Create a new folder in Outlook Express called (for example) Inbox2. Copy everything in your Inbox to that folder. (The reason for doing that is to ensure that, when you regain your old emails, you don't lose any other mail).
Close Outlook Express.
Go to Start > Search and run a search for Inbox.dbx. (You'll need to ensure that the 'Show hidden files' option is selected). Right click on the file and select 'Open containing folder'. Resize the window so that it only occupies part of your screen.
Repeat the instruction in the previous paragraph but for 'Inbox.bak' instead of 'Inbox.dbx'. (If you find more than one file called Inbox.bak, you're looking for the one which was created at the point where you compacted your files).
You should now have two windows open side by side. Right-click on Inbox.dbx and select 'Delete'. (Now you know why you copied all of your existing emails at the start - you've just deleted your current Inbox!). Right-click on Inbox.bak and select 'Rename'. Change the file name to Inbox.dbx. Drag that file across to the other open window. Restart Outlook Express. You old Inbox should have reappeared.
Chris