Upon further tinkering you don't even need to delete or move that UI file. If you edit it and delete the [MainChat] block it should reset just the main chat and keep all of your other UI positions so you don't lose any of your hard work organizing your UI. Also check to see if 'Locked' is some how stubbornly set to true if you want to try to keep your chat settings intact as well.
example:
Code:
[Main]
UISkin=velious
AtlasSkin=Default
[MainChat]
INIVersion=1
XPos1024x768=91
YPos1024x768=521
Width1024x768=683
Height1024x768=250
BGTint.red=255
BGTint.green=255
BGTint.blue=255
BGType=1
Fades=true
Delay=2000
Duration=500
Alpha=255
FadeToAlpha=255
Locked=false <---
[ChatWindow]
INIVersion=1
...