Unfortunately it doesn't look as though there is an easy way to read HighScores.dat, and while you can shrink the game window, you can't expand it beyond its initial size. You can, however, download a hex editor, and it does seem as though there is a pattern from which you can calculate your high scores:
- Columns 04 and 0C are the ones to look at. The score for level 1 seems to be stored in 00000004 (row 00000000, offset 04), level 2 in 0000000C, and so on.
- A level you haven't played will be stored as E7. One on which you've lost will be preceded by FF FF FF FF.
- A level you have played will be stored as one less than your number of turns, in hex, so completing a level in 17 turns will show up as 10; 26 will show up as 19; etc.
You've probably already tried the other themes (Penguins seems to be the only one that wouldn't cause everything to stack, not just the levels you've completed). You might try an alternate theme - there are plenty of them here on his site, but unfortunately there isn't a preview for any of them.
If you can wait for a bit, it sounds like he's planning to rewrite the game again with more modern graphics (although if you're constrained to 640x480, that may not help you much): from his games' page on Facebook:
I think it's about time I re-wrote Windows Slay from scratch using DirectX for some smoother graphics. The AI could do with a re-write too as it's not changed since 1994 and computers have got just a bit more powerful since then!