Hello.
Let's get the obvious out of the way; always delete logs after cybersex.
Now, as far as deleting some chatlogs on another pc, it is totally impossible unless you have a remote desktop software. You see, the chatlogs, although they display in a browser window, are actually files stored on the computer's hard drive.
Now, I don't actually need to tell you that you can't (or at least should not) get to play in people's files. (Just image if I could from my house access your files, that'd be hacking into a computer).
The only
legal way you can acess the computer by not being in front of it is by some remote access software. For instance, Microsoft Windows XP Pro (or Vista pro and Ultimate) will offer you the option to connect to your computer by remote distance, allowing you to be able to work on your computer as if it were in front of you.
Now, I am morally obliged to say that even if your mother's computer do have remote access and that you have her password; accessing it without her approval would probaly be wrong (at least morally).
My piece of advice; there's an option in messenger plus! to encrypt chatlog (therefore no one can open it without a password). I'd use it if I were you.