Sounds like a problem with too many programs running and causing conflicts and a drain on the resources. (Yes, too many running programs can cause issues with XP too.) First, it's probably not a real good idea to leave lots of open Windows open from multiple programs. Try to limit them as much as possible. Although XP will do a great job with multiple windows, it's not perfect.

Next, try to shut down all the unnecessary programs and services that load at start up. They're also causing conflicts and use resources. Follow the procedures in the links below and it should help:

How To Shut Down Unnecessary Start Up Programs

How To Clean Unwanted Files From The Hard Drive

Hope this helps.