I've had this happen a few times. A review of the system event log (Start / Run / eventvwr.msc / System) shows that the Macromedia Licensing Service starts, then stops. Suggestions found elsewhere to manually start this service (Start / Run / services.msc ) did not help.
I also downloaded the Macromedia License Cleaner - no help (although it *did* delete my license, requiring me to re-enter it once i got the main problem fixed.
Murry, a Team Macromedia Volunteer for Dreamweaver, suggested deleting the folder:
C:\Documents and Settings\{username}\Application Data\Macromedia\Dreamweaver
MX 2004\Configuration\SiteCache\{sitename}
where {sitename} is the site to be opened on startup (or delete them all, they aren't *that* hard to rebuild...) Just to make sure, i delete the {sitename}.dws files and the {sitename} folder.
Dreamweaver then restarts just fine, rebuilds the cache and carries on as normal.
{editorial note: Macromedia needs to fix this bug. I have had this happen at work twice in the last six months and once at home about a month or two after i upgraded to MX 2004}