Can't watch videos - help please

[basingstokett]

Help for a novice please!

I used to be able to watch video clips but now I can't. For example on

http://www.youtube.com

I get the message
"you either have JavaScript turned off or an old version of Macromedia's Flash Player. Get the latest flash player"

I've checked various things:

Java and Active X settings are enabled in Internet Options(and have now reset to Medium).
Installing Flash from the link to the Adobe site but it doesn't seem to work for me - after going through the various steps.
I've tried uninstalling the current version of Flash then try again but no luck.

Could it be something to do with my Firewall (Windows) or Anti-Virus (AVG)??? Not sure how to check and am a bit confused!

Thanks