This does seem to resolve the problem. This machine is running XP Pro... I don't know if the problem still exists on Vista platforms. For anyone just coming to the forum this works. I tried a variety of different things over the past hours, but this makes Pegasus and Norton play together again:
1) download the "symrapidreleasedefsx86.exe" file to your computer from the latest relevant folder in ftp://ftp.symantec.com/AVDEFS/norton_antivirus_corp/rapidrelease/sequence/
2) run that file. If there are problems, I'd try downloading a different version. I used the one out of the 63638 folder and it worked without problems.
3) backup your pmail folder somewhere else under a different name, just in case.
4) reinstall Pegasus. Download a fresh copy if you have errors. During install, you should be able to point to the original folder and it will give you the option to "update installation". This option will restore any missing files, but won't wreck your old mail or settings. Alternatively, all that seems to be needed is a current copy of winpm-32.exe put back into the PMAIL directory (your old shortcuts will tell you where it was supposed to be if you right-click and select "properties" on the shortcut). Old backups work for that, if Norton didn't *kindly* remove them, too.
5) if you have problems not being able to download mail, make sure your firewall settings didn't get changed to block Pegasus from internet access. With Norton Internet Security 2007, open NIS 2007 and select settings->personal firewall->configure->program control and then select "Allow" under the "Access" column next to the Pegasus entry.
BTW, I would suggest that as many of us as possible send notices to Norton to advise them that, while accidents do happen, a program as old and well-known as Pegasus Mail shouldn't get shut down like this. Kudos to the techies who got the signatures updated, finally.
Also, for anyone so inclined, PandaSoftware has some nice, solid security software offerings. I prefer the configurability of platinum, but titanium is good too.