So I using Mandriva 2007.1, I have 1Gb of ram, and I set 2Gb Swap partition.
My problem is when I using linux for couple of hours (surfing internet and using some applications) and my swap partition gets to 90% full, applications start to not responding, then everything hangs so i have to push the power off button. ctrl+alt+backspase won't work.

And even if swap is just 20% full, it hangs when I press ctrl+alt+backspace or trying to shut it down, it sounds like hdd is busy (loading or unloading something, maybe removing swap files, idk) but after several minutes still nothing happens, just black screen with not moving mouse pointer on it.

I tried "swapoff /dev/hda4" << my swap partition, but it says invalid argument.

Do you have any ideas what is this and how to make it work properly?

Any help is appreciated.

It`s too suspicious and seems like memory corruption. Can you run memtest86+ on your machine?

Moreover, you system consumes too much swap - try this as root:
It should tell you system to use less swap.

And do not use reset button - there is magick in you kernel :-) Use SySRq \Printscreen button to sync data (alt+sysqr+s), safely remount partitions as readonly(alt+sysrq+u) and reboot (alt+sysrq+b).

Swappiness doesn't (directly) affect the usage of swap on your system. It changes the bias between using RAM for applicaion memory or for (disk) cache, so for some workload mixes it may avoid swap being allocated. If something keeps acquiring storage you'll start swapping regardless.

Sounds like the OP has a memory leak somewhere - FF has been a pig until recently. If you have the sysstat package installed (and active), there will be plenty of info there to spot the suspect. Else have a look at top - sorted on VIRT or RES or SWAP would probably show. I tend to run it in the background writing to a file for later analysis, but you might be able to spot your problem on-line.

BTW, the swapoff command will always fail if all the swapped pages can't be fitted back in RAM - this is the system protecting itself.