85GB free on Mac yet "not enough space to download"?

I have 85GB free on my mac yet at the moment it's not even letting me download a single photo or anything. When I try it says "Safari can’t save the file". I have over 2GB free of memory. I don't get what the issue is? Sometimes when I restart the computer I can download a few things but then it doesn't let me download anymore.

Also, sometimes it comes up with that black message "You need to restart your computer" as soon as I log in.

And sometimes when I restart the computer the log in screen doesn't load. At times I have to restart it like 3 times before I can even get into the log in screen.

I've been having these problems the past few days. Should I start backing up everything on my mac while I can?

It's a Mac OSX 10.6.7 with a 2.4GHz Intel Core 2 Duo processor.

Please help!

 

Thanks for your reply.

It's Version 10.6.7. The model is MacBookPro7,1

Here are photos of the message that pops up when I try to download the photos. The first one is when I try to save it to the downloads folder, the second to any other place, the third one is when I try to take a screenshot. I had to take photos on my iPhone and upload them because my mac won't even allow me to save screenshots.

I don't really understand because I can still do things like surf the net, watch YouTube videos and such.

I'll try and do some of the methods/links you gave me and see how it goes.

 

Thanks for your help but it doesn't seem to be the answer. I looked at the link and it's enabled for me to "read and write" everything.

All the problems I listed in the first post started happening at the same time (Can't save any files, getting black screen telling me to restart, log in screen doesn't load sometimes after I've restarted, having to force Safari to quit constantly because I've "run out of disc space" even though I have plenty) so I think it's somehow related but I really have no idea how to fix it.