Something is seriously wrong

Hi, I really hope someone can help because after I post this I'm not even sure if I'll be able to log back into my mac, and I'm really worried because I think someone may have hacked into my mac online somehow.
Somehow when I was trying to change my password it reset it to something from the keychain - I have no idea what this is. My concern started with black frames appearing where I click the mouse, there's one around this text box now, and there will be one on my browser wherever I click, or on the desktop around whatever icons I click, including drop down menus. Also, there are now two commands on the drop down menu to shut down, restart, log off, and the screen jumps about of its own accord. This is really freaky. If anyone can shed any light on what's going on here I will be very grateful. I hope someone can tonight as I won't be able to log on without my password! Thanks

do you have a "master" password set? do you have a backup of everything? just couple things to clear off before we start messing with the system....

Hi thanks for replying! No - master password seems totally lost. Lost just now tryin to reset. Accidentally clicked something and saw a box asking if it was ok to use keychain - i didn't pay attention and hit ok, now I'm screwed because I have no idea what word it has taken. I've tried all the passwords I ever used on this mac and they are wrong.

I have a back up of everything like music and pictures and documents - but not of emails or Safari bookmarks.