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It's been a problem for many since Win7 64bit and AutoCAD 2010 (many users down to R2008 report the problem too): AFAIK there is no real fix, just workarounds like turning off Win7's Aero theme and such that work for some users but not all.
Fix worked. Autodesk "easy rebate" is asking me the part number for AutoCad LT 2012. The order number and serial number are not good enough.