Just installed SP3 on XP Home, and now videos won't play. I mean, they load and you can skip around to different parts, but they just won't play. Happens on Youtube, Myspace, Cinemassacre, and likely on anything else out there.
What can I do??

Try reinstalling the Flash plugin.

reboot

Do all of the above and if you're still having trouble, the problem is probably the audio driver. Try to get an updated one from your sound chip manufacturer.

Depending on how you've installed things, SP3 can do some odd stuff. If you're using windows drivers for sound or video then they've probably changed with the update and it'd be best to install the manufacturer ones.

If you have a codec pack (like K-lite), uninstall and grab the newest copy. SP3 appeared to conflict with it for me at least.

I thought of that initially but I wasn't sure if a codec pack could messwith streamed stuff over the net. Suppose it might if it tinkered with the video overlay in some way

I don't think Flash uses anything on the system for video decoding (except maybe for h264 which may have a few special hooks into the video driver, but that's rarely used on YouTube)... I'd rule out codec pack problems because of that.

Throwing it out just in case it's not only Flash video. Of the sites listed, I only use Youtube.
Personal experience and a little googling shows some problems with the combination of SP3, so I'm not wanting to rule it completely out.