I think I've determined the source of my XP IE8 beta 1 problem -- an excerpt from a June, 2007 Skype support blog:
Last week Microsoft emailed us and said they are getting a large number of Windows Error Reports from a recent version of the Skype add-in for Internet Explorer.
- I deeply apologize for any trouble this might be causing you
- We are working hard to find the solution to the problem
- We thank Microsoft for reaching out to us to notify us of this problem.
You now have three options for how to solve this problem:
(See the full post for more details)
Naturally I was running a stale version of Skype (who knew? I had to go to the Skype menu to check for updates...), and disabling the Skype IE add-in seems to have done the trick -- IE8 beta 1 is now very stable on my XP PC (it has been stable for a week on my Vista test PC, on which I haven't installed Skype), and, when using IE8's IE7 emulation mode, the primary difference I can discern between IE7 and IE8 is that IE8 is a lot faster.
BTW I find it perplexing that few people have complained about Skype's IE add-in, since it's essentially similar to the proposed IE6 Microsoft smart tag add-ins for which Microsoft was vilified during June, 2001 (leading to Microsoft's scrapping of its IE6 smart tag plans).