iMac totally messed up!

Okay, my iMac is totally weirded out. When I go to open any kind document, be it word document, an e-mail document, etc... it says the following:

*Insert program name here* can not open this document. It is in use by "another user". Would you like to make a copy?

Then I click okay and it says this:

*Insert program name here* can not save or create this document. The disk may be full or write-protected. Try one of the following:

1) Free up more memory (512 MB memory should do it...)

2) Be sure the disk is not full, write protected, or damaged.


I'm running mac OS X
Help! what should I do! :4-dontkno