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#21
Your rig seems somewhat old to me, and with 256MB of RAM on your GPU, I can't exactly say I'm sure you're able to play all videos smoothly, especially if they are in HD.
How long have you been using Windows 7? Have you upgraded from Vista, by any chance, or did a clean install from XP (or Vista all the same)? Have you been experiencing this problem as soon as you installed Windows 7, or is this issue a bit newer?