I can think of lots of things I would try but, since you obviously don't want lots of technical stuff, the first place would be to try System Restore.
If you've never used it before, this is how to locate it in Windows ME. (I think it's fairly similar in XP but, if not, see below): Click 'Start' and use your mouse to go to 'Progams' then 'Accessories', then 'System Tools' and, finally, click on 'System Restore'.
If you can't locate System Restore in the way I've described , click 'Start' and 'Help' and look for 'Use System Restore'.
When you eventually get to System Restore, you'll be presented with a calendar with some dates in bold. Select one of these dates (choosing the most recent one you can see when Mozilla still worked OK) and click on 'Restore to this Date'. Sit back and wait while your computer restores your system and then automatically restarts itself. Hopefully, this will solve the problem. (Note that, if you've installed any programs between the restore date and the current date, you'll have to re-install these but you won't lose any e-mails, documents, pictures, etc which you've acquired during the intervening period).
If System Restore doesn't work put another message on AnswerBank and we'll all put our heads together for you!
Best of luck!
Chris