Problems with screen freezing while playing web-based video

When I try to run .AVI or .WMV video clips via the internet, using
either IE7 or Firefox, the video does not play AND my system becomes
very choppy and unresponsive until I kill the application. Often my
screen will go black temporarily and then come back. This was a brand
new install of Windows, so I am pretty sure I have not got anything
massively corrupted. This was not a problem on my previous build, but
I didn't have WMP11 then, and I also didn't have the same version of
the nVidia drivers.

Any suggestions on how to rectify this? I am wondering specifically
about the following:

1. By installing additional codecs (see below) did I "confuse" my
browsers or cause them to select the wrong codec?
2. Are there any known issues with the nVidia drivers that I am using
(see below)?

Thanks for any suggestions.

My system:
Athlon 2700+ XP
Windows XP Pro.
1.5GB RAM
6600GT Video Card running nVidia Driver 6.14.10.9371
WMP 11 installed
DivX player installed
ffdshow codec pack installed