Thanks, Insipid. The reason I didn't get back to you is that the computer crashed and I couldn't get on line. I ended up getting another one.
I e-mailed the Gary Null webmaster who said this problem was caused by a hacker on their site (pogo) and that bookmarking a page hosted by them might be causing it. I don't remember ever visiting pogo, but they said they have numerous pages and that some have changed hands so it could be a lot of different pages. I e-mailed their tech support and they said they don't help with problems outside of their own software.
Then I found a discussion forum which said this had been happening with people browsing using the 2004 version of Firefox. That's what I'd used. I got rid of any bookmarkes that had been bookmarked since before the problem and after about 5 hours on line I didn't see Gary Null again. Then I ran 5 different scans to be sure the computer was clean and downloaded the XP service pack 2. It crashed the computer and I couldn't even start it up to get into safe mode to do a system restore.
Then I took it into the nearest town to a guy who has an excellent reputation for fixing computers and he really had to work to save my data. There must have been some problems with the computer before, because it didn't let me save anything to a CD or floppy. I tried to save data before downloading the SP2. It would have cost more than my Dell Dimension 2100 (4+ years old) was worth, so he put my data into another computer and put in another (don't remember what it's called) so now I have a D drive too and can back things up to it in case the other one crashes.
The fan on this computer is noisy and I'll be taking it in to be repaired this morning. If I don't get back to you again soon it's because the computer is at the shop.
Thanks for trying to help.