A few weeks ago I wrote:

"I'm running Firebird 1.5.x SuperServer on Windows XP and it mostly
runs great.
Notably, 1.5 runs significantly better (much faster connects and
disconnects) on our XP machine than 1.0 did.
However, I'm still having an annoying problem.

After the server has been up for a couple days, we start getting calls
from users saying they can't connect to the server. When in this
state, neither our Internet users nor our intranet users can connect.
IBConsole reports, "Cannot attach to services manager."

InterClient throws the following exception:
interbase.interclient.UnavailableInterBaseServerException:
[interclient][interbase] unavailable database
See API reference for exception
interbase.interclient.UnavailableInterBaseServer

I found a borland site that indicates this error means the server has
stopped running, but when I look at the services list it reports
Firebird as running.

Any ideas?"

Apparently nobody had any :(

I've researched the problem a little more and have found that in my
event log there is repeatedly an event 251 coming from
FirebirdGuardianDefaultInstance. I'm not sure what this event
indicates, but it seems to have some relation to the database
unavailable message. I found other references on a Google search to
event 251 and database unavailable errors, but no answers.

Another bit of information is that ever since I upgraded to Firebird
1.5, when I start IBConsole it says, "The server has not been started.
Would you like to start it now?" When I click yes, it comes back
with, "Interbase Server is not installed on your system." But, then
when I enter my username/password it connects anyway!

This all seems to be indicative of something, but I'm having trouble
putting the pieces together.

Mike