If both IE and FF are causing errors, my first guess would be that the Flash player may be corrupted. You can try viewing some other Flash files on other sites, however, such as the Flash and Shockwave pages on Adobe's site, and see if they work.
The YouTube warning page is the same whether the problem is with Flash or with the security settings not allowing JS to run. Since the FF and IE browsers each have their own JS implementations, the problem is more likely with Flash than JS. Also, IE and FF each maintain their own independent security settings, so it's not likely a security setting problem in those programs; and they maintain their own caches, so a corrupted file stuck in the cache isn't the likely cause, either.
Some other possibilities...
- A firewall setting (internal or external) is preventing Flash from accessing the Internet (unlikely, but possible).
- A setting within the Flash player itself is corrupted and not letting it access the Internet (even less likely).
- You have some sort of malware LSP that is preventing the Flash player from accessing the Internet (haven't come across this, but with hundreds of new pieces of malware coming out every day, it's possible).
So... I would first check the firewall settings to see if the Flash Player is blocked from Internet access.
If not, then I would try completely uninstalling and reinstalling the Flash Player (with a reboot in between, just to make sure the RAM is clear).
If not that, I would use something like LSPFix or HijackThis to look for extra LSP's that don't belong there. (This is sort of advanced, so if it sounds like Greek, don't do it.)
You also may want to try running Crap Cleaner (
http://www.ccleaner.com ) including the Registry cleaner, and then rebooting. Make a backup (you will be prompted), just in case. CCleaner is one of the better automated registry tools, but you never know.
If none of these work, please post again.
Best,
Rich
This would be Chip's cue for a comment about that big pretty apple. 
