Have you tried changing* your password? If someone else knows it, they can be logged in at the same time as you an contribute to the chat as you. This would not change something you had already sent.
If you mean someone is deleting parts of what you have already typed and sent, and replacing it with something else, that would be someone editing your chat history.
Skype used to store chat history for each computer locally (in the user profile - Application Data) so someone with remote access to your files could change your history, but more recently I thought it was synced via the cloud / p2p as contacts are.
If you have any Skype Extras / Extensions added, there might be something dodgy in there that could manipulate your chat.
If someone has a backdoor into your computer such that they can send keypresses remotely, then all bets are off as they could type in any program. Legitimate remote access software such as VNC can also do this.
(*) Given what you have described, I would use a different computer from the one where you see this problem, and go directly to https://login.skype.com/
to sign in and change your password.
prompt $P - Invalid drive specification - Abort, Retry, Fail? $G
prlzx on n e w n e t Max ADSL
Edited by prlzx (Sat 06-Aug-11 19:39:36)