Since I have upgraded my fc3 machine to fc4 when I work some programs simultaneously every thing get frozen and I have to reset my computer manually. I have a athlon 2000 XP+ with 384 MB ram. Swap file is 777MB but it never works for me. In system monitoring it is always empty even when run huge program.

Are you actually activating it? Have you got it in /etc/fstab
so you get it switched on automagically? And is it actually
a file or have you got a swap-partition (you should)? Where
is it set-up, how has it been set-up?

swapon <filename>


Cheers,
Tink

Simply issue "swapon -s" to see if you have any swap enabled/active.

/sbin/swapon -s
Filename Type Size Used Priority
/dev/hda3 partition 795208 900 -1

swap file is available in fstab but I don't know why it never works for me. when my CPU usage become very high my computer freezes and every thing gets stopped and I have to reset it

Please post the swap partition entry from /etc/fstab.

# This file is edited by fstab-sync - see 'man fstab-sync' for details
LABEL=/ / ext3 defaults 1 1
/dev/devpts /dev/pts devpts gid=5,mode=620 0 0
/dev/shm /dev/shm tmpfs defaults 0 0
LABEL=/home /home ext3 defaults 1 2
/dev/proc /proc proc defaults 0 0
/dev/sys /sys sysfs defaults 0 0
LABEL=SWAP-hda3 swap swap defaults 0 0

I am not sure whether the problem is with swap file. Maybe my athlon XP 2000+ is not good enough for linux

Umm, yeah, it's "good enough for Linux." That isn't the problem.

The swapon output indicates that the swap-partition is being used, although it is almost completely empty. I conclude that whatever is causing your system to freeze is not related to swapping.

Does /var/log/messages have anything?

What's the output of free?

In terms of the labeling - my understanding was that
the kind of label you're using is specific to ext2/3, and am
not certain that it will work with swap files. Try something
like this instead:
/dev/hda3 swap swap defaults 0 0


Cheers,
Tink

in /var/log/messages there is nothing written about this problem

I changed the fstab to /dev/hda3 swap swap defaults 0 0

I hope it works

I still have that problem.
Even when I am reading a text and doiing nothing special my computer freezes.

in the system monitor when it freezes CPU usage is 100%

What does dmesg have to say at the point it attempts to mount the swap ???>

at the end of the file is written:
Adding 795208k swap on /dev/hda3. Priority:-1 extents:1

But I don't think that anything's wrong with swap
I have doubts about FAN, because CPU temperature is around 80c and system is 44c

I was working on windows and my computer suddenly stopped working. that means there is a problem in hardware.
I have 2 RAMs. one 128 and the other 256.i think one of them doesn't work properly.
I have doubt about my CPU fan also. Since I have an AMD athlon machine it heats so moch and i think my fan is not enough for it.
I don't think that there could be any other errors.