Computer Videos Freezing and Halting but Sound is Fine?

Whenever I download videos onto my hard drive, they play at a halting pace. The sound will work well, continuing uninterrupted, but the video images start and stop catching up with the words of the characters in the shows. I have increased my Virtual Memory thinking that it might be due to a paucity of it but that hasn't had any real impact. I have also experimented by watching the videos in VLC and in Windows Media Explorer. None of that has made any difference. I am thinking that perhaps I should buy more RAM but that might not have any impact if increasing Virtual RAM didn't have any positive impact. Can anyone offer me some advice on what to do?

Note that this does not happen to streaming videos as much. The problem I am speaking of happens to videos that have been downloaded to my hard drive.

Below are my computer specifications:
Toshiba Computer Satellite L645D
Processor: AMD Phenom II P820 Triple Core Processor 1.80 Ghz
Installed RAM: 4.00 GB (3.74 Usable)
Windows 7 64-bit operating system
Display Device: ATI Mobility Radeon HD 4200 Series
ATI Technologies Inc.
ATI Display Adaptor
Internal DAC (400 Mhz)
Approx Total Memory: 1913 MB