Problems with Sound.

Help or advice needed.
Windows XP Home SP2 Intel Celeron CPU 2.60GHz
224 mb of RAM.
A month ago overnite all sound konked out on my system. Everything checked
out - connections, speakers, et al. By fiddling over the next few days, I got
about half of the sound back on. Following is current:
DVD on Windows Media plays sound and video good.
CD's music on Windows plays good.
BBC video clips picture and sound good on Media. BBC radio works fine.
thatvideosite.com sound and picture okay - video a little jerky.
funnyvideos.co.uk - sound good.
bofunk.com - sound good.
m90.org - sound good.

No sound when Windows opens.
No sound when Incredimail notifier comes on.
Ecards - all types - no sound.
Google videos - picture good, no sound.
YouTube.com - picture good, no sound at all.
Firefox video clips - picture good, no sound at all.
mojoflix.com - no sound.
filecobi.net - no sound at all.

Device Manager - System speaker - no drivers are installed for this device.
- Drivers Microsoft 7/1/2001 Version 5.1.2600.0
- No driver files have been loaded
Update Wizard - no drivers to update or cannot find.
No device installed.

That last baffles me as a device (speakers) are installed and operating. Is
there any significance to the fact some sites work (i.e. UK sites) and not
others?
Hope this info is sufficient to explain the problem. Thanks for any info.