I think the source of the pop-up windows that many visitors have seen was one of my website counters (Motigo). I've removed it, and hope this fixes the problem. Thanks for all the helpful suggestions. If you still see popups, however, please let me know.
In any event, it bears repeating that using a browser like Internet Explorer, or any browser that does not give you the option of blocking popups, exposes you to the annoyance of popup windows. I highly recommend using either Safari or Firefox (I use both).