PC freezes,can't find problem

This past week or two my PC has been freezing when using certain programs,usually when encoding DVD's-video files.. I don't know what has changed to make it start doing this. The programs it happens to are DVD Shrink 3.2 when backing up a DVD (It dosent affect DVD Decrypter though,in read or write mode!) TMPGenc plus 2.5 when encoding video TMPGenc DVD Author 1.6 when outputting project to a Video TSAudio TS Folder

Although today,it froze when only Outlook Express was open.I went to the other room for a while,and came back the PC had froze.The only way out of it is to reset the PC

I have 2 x 256 meg RAM modules,so i tried each of them individually to see if it made a difference,maybe to find bad ram,but it still happened. I tried turning off NAV NIS too,but still got same problem.Also,when the PC restarts,sometimes i get the blue screen of rest just before it gets to the welcome screen,and it restarts again.

The CPU seems able to handle a lot of tasks,i opened 4 different video players all at once playing different video files,and they all worked perfectly at the same time. Having said that,when i use any of the above mentioned programs,i only have that one program open with nothing else running.

Any ideas what else i can check? I have a new hard drive,new Gigabyte mobo in a new case with other bits i transferred from my old PC. I have some DVD's to make and i cant do them til i get this sorted out. thanks