the drive letters that assigned by Windows will basing on drive letter priority, and the letters priority will be bases on primary partitions that exists on every physical drives, active partition(s), the extended partitions and logical drives. Windows also record all the changes that the Windows system and you have made to the mounted devices in registry.
it's possible and quite easy to remove the registry and to have Windows reassigns the drive letters to all drives, however, if you used to assign some drive letters with some incorrect drive letter priority (and as you may have experienced before, that Windows has honored the drive letters settings), then you may have some unexpected problems with the drive letters changes.
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speaking of Opera, the Opera 9.5
alpha release...