AFAIK this only happens on Windows based NetBackup servers.
Sounds to me like someone has removed/deleted or renamed/moved one or more core file(s) of NetBackup.
When NetBackup cannot find a file that it expects the WIndows Installer gets called and this shuts down NetBackup.
I had exact same problem, and we found that someone had renamed one of the "notify" command files.
We found that we didn't have a file called:
<install-path>\NetBackup\bin\userreq_notify.cmd
....but we did have a file called:
<install-path>\NetBackup\bin\user_req_notify.cmd
...note the extra "_" underscore between "user" and "req".
So, all we did was rename the file back to what it should be (after checking the contents of course) and problem went away.
If your problem isn't to do with "userreq" then I would supose another file may be missing.
I'd check through the sys/app/sec event logs for more details maybe.
Regards,
Dave.