FSekrit should run just fine on anything from Win2000 up.
Actually from Win9x and up - I had to go through a few pains to make that work (and then there were a couple of versions where I accidentally botched the win9x support :$).
So, no, there should be nothing preventing fSekrit working on XP. In fact, I've just tested 1.40 on XP-SP3 (under vmware), and it runs just fine. fSekrit also doesn't use any of the fancy instruction sets, so in theory you should be able to run it on... dunno, probably even a 386
Please do try getting a capture of the error message. Perhaps antivirus? Some of those products don't like fSekrit, either because it's compressed with PECompact, or because of heuristics (copying itself to %TEMP% and launching from there, writing to it's own executable).
c.gingerich, from your site: "256 bit RSA_AES encryption with password". That seems like a weird encryption scheme to choose for an application like this?