NVidia Media Centre is part of the Detonator Drivers.
Right click desktop, select Properties, Settings, Advanced. Select your NVidia Card Tab. Go down the Desktop Utilities and Uncheck Show Display media Centre icon in the Taskbar.
Also try disabling your AV as it maybe scanning your in-boxes whilst it opens.
Also compact your folder in OE and remove all unneeded messages esp those with massive attachments. You can save the attachment to a folder on your PC and delete the messages. Clear the Deleted items too... then compact the folders - Tools menu, Options, Maintainance, Clean Up Now, Compact