This pc config is

Pentium 4 3Ghz
1 GB of RAM
20 Gb of HD

When installing linux i decided that i should not have swap.

So, yesterday i was editing a pic on Gimp and hearing some mp3 on Xmms. I realized that the computer (up for 24 hours) was really slow. Them, gimp crashed. I tried to open it again, but nothing happened. At this point, even the mouse movie was slow.

I closed kde and saw some errors messages saying that my hard disk was full. But i have 1 gb that the system use and 5 gb of mp3.

When i tried df on console. 100% of usage on my hd

What steps should i follow to detect what happened?

Is there a *nix virus that make this?

When i download a file, how can i force linux to auto change permissions to 400 (is this the best chmod?)

Look for a core file saved when gimp crashed.

If there are core files present, you can delete them. The ulimit command can stop core files from being created, or limit their size.

Not having any swap may not be a good idea.

Someone or something is joking with me. Right now, there is 58% of hard disk use (which is the normal, i guess).

Could this 42% of files be cache files or something like this?

Check the /tmp directory for any huge tmp files.

Are you sure you've disabled swapping? I think you've simply told the swapper to use your normal partition for swapping instead of a dedicated swap partition.
By doing so, swap can use up much of your disk space if your programs need huge amounts of memory.

Having a dedicated swap partition limits the disk space used for swapping. Making it too small may cause your system to slow down too.

How much RAM do you have?

I have 1 gb of RAM. How do i check if it was a swap file?

scuzzman

I´ll check it as soon as i get on that computer

cat /proc/swaps

will list any swaps you have currently.

Also look into the du command to see how much space each file is using.

Like int0x80 said , go to /
and issue

Actually, in Unix it's usually a swap partition eg see
cat /etc/fstab