Sound problems - only partial sound.

Windows XP Home SP2. Some time ago my sound konked out completely overnight.
Over the next few days I fiddled and got about half of it working.
The following is the way things stand now:
DVD on Windows Media play sound and video good. CD's play good.
BBC video clips - sound good on Media. BBC radio good.
thatvideosite.com - sound and picture good.
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.
E-cards - 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.

This 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 and any help or
advice is appreciated.

Exeter.