It is possible that this is due to viruses in your system, so check ur full system with good anti-virus. I suggest, for MWAV (not escan & it is completely free) for emergency scan.
One more thing might be, occurred when another program has registered itself as the default application for browsing the web. If this program is not well written, or it has been uninstalled, Outlook Express cannot launch a browser window when you click on a link within an email. To overcome this issue, must re-register Internet Explorer as the default browser. To do this:
Open IE and click Tools >Internet Options menu > Programs tab
Look for a checkbox where “Internet Explorer should check to see whether it is the default browser”, If this checkbox is not ticked, checked it, now click OK. Close IE and again open IE
and close Outlook Express and re open Outlook Express…..
For complete info : http://support.microsoft.com/kb/929867
Also. Try a fresh install of outlook express. That usually works
“Also. Try a fresh install of outlook….”, this is not a good idea. A computer has problem, that does not mean to format it first, without taking other considerations.