I was having the same problem. It took me a while to find out where my Media Cache files were being stored… they were taking up over 70 gigs!
I have premiere 5.5 on a mac. I found the cache files in
Library>Application support>adobe>common>media cache files
Empty it out!
In my case, I now have to wait a while when I open up the project, it seems to have to conform the files, which is taking some time. But at least I can save my projects now.
Next, I will try in the SEQUENCE menu to click on DELETE RENDER FILES from time to time… that should help too.