Kodika:
I am sorry I am not able to tell you what is causing the issue you are experiencing. I don't know whether you have made any recent changes to your system, experienced any issues, run any maintenance or repair procedures etc. However, it is a wonder that you are able to view any videos at all with 128 MB of RAM.
I sugest that you try
starting up in Safe Mode and see if that helps.
Otherwise you might try
Repair Disk
Insert Installer disk and Restart, holding down the "C" key until grey Apple appears.
Go to Installer menu (Panther and earlier) or Utilities menu (Tiger) and launch Disk Utility.
Select your HDD (manufacturer ID) in the left panel.
Select First Aid in the Main panel.
(Check S.M.A.R.T Status of HDD at the bottom of right panel, and report if it says anything but
Verified)
Click Repair Disk on the bottom right.
If DU reports disk does not need repairs quit DU and restart.
If DU reports errors Repair again and again until DU reports disk is repaired.
If DU reports errors it cannot repair you will need to use a utility like Tech Tool Pro or Disk Warrior
Good luck.
cornelius