Not sure if this has happened to anyone else, but when I upgraded from 0.7.19 Rev 2 to the 0.8.1 Miranda Portable no longer loads.
The install completes successfully, but then when I launch the app it crashes immediately.
I've used dbtool to make sure my profile is in good order.
Try installing it into a different folder, without your current profile or plugins.
I use 0.8.1 portable here, on several machines without issues.
Best guess would be that there is an incompatible addon in your 0.7.19 install.
It ended up being the SmileyAdd plugin causing the problem. I updated the plugin to the most recent version and now 0.8.1 loads my profile without any problems.