Solved it myself, using info from http://community.pmail.com/forums/2551/ShowThread.aspx: I've found pmail.ini, where the temporary folder is actually written to be

Temporary files directory                 = C:\Windows\Temp\Pegasus

Now although C:\Windows\Temp was no problem, the \Pegasus subfolder wasn't there anymore, and apparently Pegasus doesn't create a fresh one automatically.

Sorry if I bothered anyone - and hopefully other people will benefit of the problem + solution sometime. 

Hello forum,

I've just started to receive errors on Pegasus startup: it feels my temp folders are not set correctly. I sure want to try and repair the problem for Pegasus (especially as it now closes without further warning whenever I try to open an e-mail message), but I have no clue how to.

 
- the environment variables in Control Panel / System etc. were C:\Windows\Temp before, and they were the same in Pegasus About / Info.

- I've made a new folder C:\Temp and set the environment variables accordingly; in Pegasus too, it is now C:\Temp.

- I have read and write access to C:\(Windows\)Temp and am the admin user on my computer.

I have Windows XP; I believe the problem started to occur when earlier today I ran McAfee QuickClean, which deleted most of my temporary files. When I first found that Pegasus was giving problems, I noticed I had both a C:\Windows\Temp and a C:\Windows\Temo folder; I have deleted the second after moving all of its files (which apparently were not in use, as I got no error messages) to the first. I have restarted the computer after those actions, but it didn't help.

 
Any suggestions? All help is most welcome!