Videos in IE8

Running 3GHz CPU, 2 G RAM, WinXP Pro SP3, hi speed internet (Comcast)

IE8 has been working just fine on my computer for months. IE8 about a month
ago started not loading videos from *some* sites like Marke****ch.com. It
does load YouTube and videos from other sites. I've contacted Marke****ch
and they have no answer. IE8 works just fine on my wife's computer. I opened
IE8 to tools/internet options on both my computer and my wife's and compared
the settings for all tabs and they are exactly the same, but her IE8 works
and mine doesn't. I have the latest version of flash and Java. I installed
Microsoft Silverlight 4.0.51204.0 to no avail. I disabled Windows firewall
and Avast to no avail and enabled them again. I re-booted in Safe Mode with
Networking and got the same bad results. I did not have IE8 in my Add/Remove
list as it was installed when I installed SP3. I downloaded IE8 and
installed it over the one that was there to no avail. Now IE8 shows in my
Add/Remove list, but it can not be un-installed so I can try a repair.

What could be my problem please and how do I fix it?

Henry Knoll