Here’s the latest information I received from Skype tech support.
The procedure for backing up your history files on a Windows XP machine is the following:
Locate the folder “Application Data\Skype\<your username>” and copy it to another location such as a CD or DVD-ROM.
To restore the history files just copy the files back to that folder.
These files are stored as .dbb files and can only be read with Skype itself. Unfortunately you cannot recover from a corrupt history file.
Be aware that the above files contain all of your chat room history files. You cannot save separate chat room history using that method.
If you want to preserve the history of one specific chat for reading it later, type “/htmlhistory” (without the quotes) into the chat text area and press enter. The entire chat history will open in an internet browser. You can save it for reviewing later by selecting “File > Save Page As”.