Online Videos

Don't know where else to put it...

When watching videos on my dad's computer online, the video gets choppy (like a game with really low fps) but only happens when fullscreened. On a normal minimized screen (like Youtube's) the video runs just fine.

Here's why I'm stuck:

Internet: 6 mbps, split through a router to 4 computers and we play videos just fine on the other three computers.

Computer: 512 MB, 2.9 GHz Celeron D, Geforce 4 -- something

The internet's fine since we can all play just fine. I'm also quite sure the computer's fine too since I've managed to pair up the old card with a 22 in. widescreen and a 17" CRT while playing a DVD on the 22 in". We have no firewalls, nothing networking related except for the router and modem. And we use Mozilla Firefox 2. Any other ideas to fix this "issue"?

Note that this issue applies to every online streaming sites.

The other computers are both worst and better, videos run fine on either of the three.

I've downloaded all the latest flash player updates. As for mine, mine's fine and I don't recall making any changes as it just worked. The other two I don't know.

Specs wise his would be somewhere in the middle.