..when an edit box has a signifigant (4 KB) amount of text in it?

I've been working on this problem with a Windows XP machine (strangely enough, only that machine exhibits this problem) where the user of my program tries to spawn a new window after opening a > 4 KB file. If they try this, the new process starts, but nothing appears - the new Editor.exe process enters a race condition where it chews up CPU time and memory, but doesn't display anything.

Any ideas?