The solution I've had to employ is to close down some of the lesser-needed windows and then try again, usually with success. It's as if there's a memory leak, but there's no sign of any lack of free memory.
This mystery plagued me for so long until I eventually found the cause and a fix.. An internal memory heap needed to be expanded to accommodate all these windows!
It involves a registry tweak and a reboot, but microsoft have provided a foolproof fix to run, which I used and it's finally removed the only problem I've had with XP. Wish I had found it years ago.. argh!
http://support.microsoft.com/kb/126962 (Scroll down and click on "Fix It")
Install, reboot, open all the windows you want without fear of hitting any hidden limits. (well, eventually maybe!)
