Settings always reset after restart! Help please????????????????????????????????????????????????????????????????

Basically I bought an iMac last October, 21", 4GB standard manufactured settings ram etc. After a few weeks every time I quit an application and go back on it, it returns to its old settings. Whenever I remove a bookmark from the toolbar on Safari and restart it, it returns to the old settings, when ever I restart the computer the dock and background changes towards its original settings. This is starting to get really annoying, and I don't know what to do and I have installed all available updates. Is an issue like this covered in a warranty and do you guys have any ideas on what I should do?