are you sure it isn't actually running and what's flashing on the screen is "already running"?

I run pgsql on linux but from what I read in posts about windows setups is that it is generally a start up service meaning you wouldn't be trying to run it manually.
if that's not the case for you then probably easiest is to look at the end of postgresql.log and see what it's complaining about (iirc flat out errs always log but if necessary you can bump verbosity in postgresql.conf and try again).
if that doesn't work (this is a linux path you'll need to translate yourself) then look at the file "/var/lib/pgsql/data/postmaster.opts" which shows how it was last started.
and last but not least, if none of those work, in linux it's generically started with "/usr/local/bin/postmaster -p 5432 -D /var/lib/pgsql/data -i" by postgres user (and again you'll have to make the windows path translations).