It's strange, it worked for a while and now it doesn't work anymore. I contacted help desk and here is what I got but still doesn't work.
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Please try to modify the width parameter of the <table> tag in the following templates:
1. skin1/rectangle_top.tpl
replace width="{$width|default:"100%"}"
with width="980"
2. skin1/head.tpl
replace <table cellpadding="0" cellspacing="0" width="100%">
with <table cellpadding="0" cellspacing="0" width="980">
3. skin1/customer/home.tpl
replace <table width="100%" cellpadding="0" cellspacing="0">
with <table width="980" cellpadding="0" cellspacing="0">
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I couldn't even find the parameter in home.tpl!!
Anyone can help, please?